Method for forming electrode, electrode, storage battery, and electric device

ABSTRACT

An electrode improved for achieving a storage battery having both a high electrode strength and favorable electrode conductivity is provided. The electrode includes graphene and a modified polymer in an active material layer or includes a layer substantially formed of carbon particles and an active material layer including a modified polymer over a current collector. The modified polymer has a poly(vinylidene fluoride) structure and partly has a polyene structure or an aromatic ring structure. The polyene structure or the aromatic ring structure is sandwiched between poly(vinylidene fluoride) structures.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the present invention relates to a method for forminga storage battery electrode.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. In addition, one embodimentof the present invention relates to a process, a machine, manufacture,or a composition of matter. Specifically, examples of the technicalfield of one embodiment of the present invention disclosed in thisspecification include a semiconductor device, a display device, alight-emitting device, a power storage device, a memory device, a methodfor driving any of them, and a method for manufacturing any of them.

2. Description of the Related Art

With the recent rapid spread of portable electronic devices such asmobile phones, smartphones, electronic book (e-book) readers, andportable game machines, secondary batteries for drive power sources havebeen increasingly required to be smaller and to have higher capacity.Storage batteries typified by lithium-ion secondary batteries, whichhave advantages such as high energy density and high capacity, have beenwidely used as secondary batteries for portable electronic devices.

A lithium-ion secondary battery, which is one of storage batteries andwidely used due to its high energy density, includes a positiveelectrode including an active material such as lithium cobalt oxide(LiCoO₂) or lithium iron phosphate (LiFePO₄), a negative electrodeformed of a carbon material such as graphite capable of reception andrelease of lithium ions, a nonaqueous electrolytic solution in which asupporting electrolyte formed of a lithium salt such as LiBF₄ or LiPF₆is dissolved in an organic solvent such as ethylene carbonate or diethylcarbonate, and the like. The lithium-ion secondary battery is chargedand discharged in such a way that lithium ions in the secondary batterymove between the positive electrode and the negative electrode throughthe nonaqueous electrolytic solution and inserted into or extracted fromthe active materials of the positive electrode and the negativeelectrode. In particular, Patent Document 1 is very highly evaluated asa document that discloses a rocking-chair lithium-ion battery.

Here, the structure of a commonly used battery electrode is described.The electrode includes, over a current collector that is a conductivesupport, an active material layer which includes inorganic compoundparticles serving as an active material and a binder for keeping theshape of the electrode. As the binder, an organic polymer is used. Sincethe binder needs to be electrochemically stable, poly(vinylidenefluoride) (PVdF), which is a ferroelectric polymer, or styrene-butadienerubber (SBR), which is one kind of synthetic rubber, are often used.Furthermore, in order to apply a voltage evenly to the electrode, aconductive additive is added to the active material layer. As theconductive additive, a carbon material, especially carbon black ischiefly used in terms of electrochemical stability. The active materialparticles included in the active material layer that stably keeps theshape owing to the binder, the conductive additive, and the like areelectrically connected in parallel to each other.

However, the materials that are known to be stably available as a binderare insulators, and the carbon material that is used as the conductiveadditive has little binding force. The electrode is required to haveholes for including an electrolytic solution that transfers carrier ionsor to have a certain degree of flexibility. Furthermore, if the activematerial layer is made thick enough for high capacity, various ohmresistances are applied and the reaction of the active material in theelectrode tends to be uneven.

In view of these problems, in a known technique, the shape of a positiveelectrode where a high voltage is applied and the oxidation reactiontends to occur is stabilized and the conductivity thereof is secured byusing PVdF, which is a ferroelectric polymer, as a binder and carbonblack with a small particle diameter, typically acetylene black, as aconductive additive.

For example, Patent Document 2 discloses a technique for preventing thepromotion of gelling of PVdF due to carbon black. In addition, PatentDocument 3 discloses a combination of Ketjen Black (registeredtrademark), which is carbon black having a higher order structure, andPVdF or a copolymer including PVdF.

REFERENCE Patent Document

-   [Patent Document 1] U.S. Reissue Pat. No. 4,668,595-   [Patent Document 2] U.S. Pat. No. 6,200,703-   [Patent Document 3] United States Patent Application Publication No.    2010/0266882

SUMMARY OF THE INVENTION

With the recent energy demand, further improvements in performance, thatis, higher bonding force and lower electrode resistance, have beenstrongly required.

In consideration of the above problems, an object of one embodiment ofthe present invention is to provide a method for forming a storagebattery electrode having an interaction between a binder and aconductive additive. An object is to provide a storage battery having ahigher capacity per unit volume of an electrode with the use of astorage battery electrode formed by the above formation method.

In consideration of the above problems, an object of one embodiment ofthe present invention is to increase the electrode strength by modifyinga polymer having a poly(vinylidene fluoride) structure in an activematerial layer and thereby increasing an interaction with grapheneincluding oxygen. Another object is to reduce electrode resistance bymodifying a polymer having a poly(vinylidene fluoride) structure andforming a composite material of the polymer and graphene includingoxygen. Another object is to improve cycle performance by providing astorage battery including the above-described electrode. Note that themodification refers to elimination of hydrogen fluoride from thepoly(vinylidene fluoride) structure. The modified structure refers to apolyene structure or an aromatic ring structure.

In consideration of the above problems, an object of one embodiment ofthe present invention is to modify a polymer having a poly(vinylidenefluoride) structure that is included in an electrode and therebyincrease an interaction between the polymer and a carbon material havingbeen subjected to reduction treatment. Furthermore, another object is toreduce electrode resistance by modifying a polymer having apoly(vinylidene fluoride) structure and forming a composite material ofthe modified polymer and a carbon material having been subjected totreatment with a reducing agent. Another object is to reduce interfaceresistance between a current collector and an active material layer andimprove interface bonding strength by modifying a polymer having apoly(vinylidene fluoride) structure and performing treatment with areducing agent on a carbon material in contact with the currentcollector. Note that the modification refers to elimination of hydrogenfluoride from the poly(vinylidene fluoride) structure. The modifiedstructure refers to a polyene structure or an aromatic ring structure.

In consideration of the above problems, an object of one embodiment ofthe present invention is to provide a storage battery electrode thatuses a polymer having a modified poly(vinylidene fluoride) structure andgraphene including oxygen. Another object is to provide an electrodeincluding an active material layer including a polymer having a modifiedpoly(vinylidene fluoride) structure, a carbon black layer, and a currentcollector in contact with the carbon black layer. Another object is toprovide a storage battery including the above-described electrode.

In consideration of the above problems, an object of one embodiment ofthe present invention is to provide a method for forming an electrodewith improved strength by modifying a polymer having a poly(vinylidenefluoride) structure that is included in the electrode. Another object isto provide a method for forming an electrode having an improvedelectrode strength and a uniform electrode conductive path by performingreduction treatment on an electrode including a polymer having apoly(vinylidene fluoride) structure and graphene oxide. Another objectis to provide a method for forming an electrode with improved bondingstrength between a current collector and an active material layer byusing a carbon black layer suitable for reduction treatment, a currentcollector in contact with the carbon black layer, and a reducing agentin appropriate combination.

Graphene is a carbon material having a crystal structure in whichhexagonal skeletons of carbon are spread in a planar form and is oneatomic plane extracted from graphite crystals. Due to its electrical,mechanical, or chemical characteristics which are surprisinglyexcellent, graphene has been expected to be used for a variety of fieldsof, for example, field-effect transistors with high mobility, highlysensitive sensors, highly-efficient solar cells, and next-generationtransparent conductive films and has attracted a great deal ofattention.

Note that graphene in this specification refers to single-layer graphemeor multilayer graphene including two or more and hundred or less layers.Single-layer grapheme refers to a one-atom-thick sheet of carbonmolecules having π bonds. One sheet of graphene is referred to as agraphene flake. In addition, graphene oxide refers to a compound formedby oxidation of such graphene. When graphene oxide is reduced to formgraphene (this is also referred to as elimination reaction in graphemeoxide), oxygen included in the grapheme oxide is not entirely eliminatedand part of the oxygen remains in the grapheme, in some cases. Whengraphene includes oxygen, the ratio of oxygen measured by XPS ingraphene is higher than or equal to 2 atomic % and lower than or equalto 20 atomic %, preferably higher than or equal to 3 atomic % and lowerthan or equal to 15 atomic %. Note that reduced grapheme oxide (RGO) isalso included in grapheme.

In consideration of the above problems, an object of one embodiment ofthe present invention is to provide a novel electrode, a novel secondarybattery, or a novel power storage device. Note that the descriptions ofthese objects do not disturb the existence of other objects. In oneembodiment of the present invention, there is no need to achieve all ofthese objects. Other objects will be apparent from and can be derivedfrom the description of the specification, the drawings, the claims, andthe like.

In view of the above, one embodiment of the present invention is anelectrode which includes an active material particle, graphene, and apolymer having a poly(vinylidene fluoride) structure. In the electrode,the polymer having a poly(vinylidene fluoride) structure partly has apolyene structure or an aromatic ring structure.

One embodiment of the present invention is an electrode which includesan active material layer including an active material particle and apolymer having a poly(vinylidene fluoride) structure, and a currentcollector including a metal foil and a layer substantially formed of acarbon particle. In the electrode, the polymer having a poly(vinylidenefluoride) structure partly has a polyene structure or an aromatic ringstructure.

One embodiment of the present invention is an electrode which includesan active material layer including an active material particle,graphene, and a polymer having a poly(vinylidene fluoride) structure,and a current collector including a metal foil and a layer substantiallyformed of a carbon particle. In the electrode, the polymer having apoly(vinylidene fluoride) structure partly has a polyene structure or anaromatic ring structure.

One embodiment of the present invention is the electrode having any ofthe above-described structures, in which the polyene structure or thearomatic ring structure is provided between two poly(vinylidenefluoride) structures.

One embodiment of the present invention is the electrode having any ofthe above-described structures, in which graphene includes oxygen andthe ratio of oxygen measured by XPS in graphene is higher than or equalto 1 atomic % and lower than or equal to 20 atomic %

One embodiment of the present invention is a storage battery whichincludes the electrode having any of the above-described structures andan electrolyte.

One embodiment of the present invention is an electric device whichincludes the storage battery having the above-described structure, and ahousing, a display device, or a switch.

One embodiment of the present invention is a method for forming anelectrode which includes the steps of forming an active material layerincluding an active material particle, graphene oxide, and a polymerhaving a poly(vinylidene fluoride) structure over a current collector;and immersing the current collector in an aqueous solution including areducing agent to modify the polymer having a poly(vinylidene fluoride)structure by eliminating hydrogen fluoride.

One embodiment of the present invention is a method for forming anelectrode, which includes the steps of forming a current collector byproviding a layer substantially formed of a carbon particle over a metalfoil, forming an active material layer including graphene oxide and apolymer having a poly(vinylidene fluoride) structure over the currentcollector, and immersing the current collector in an aqueous solutionincluding a reducing agent to modify the polymer having apoly(vinylidene fluoride) structure by eliminating hydrogen fluoride.

One embodiment of the present invention is the method for forming anelectrode with any of the above-described structures, in which thereducing agent is a material having a LUMO level of higher than or equalto −5.0 eV and lower than or equal to −3.8 eV.

One embodiment of the present invention is the method for forming anelectrode in any of the above-described structures, in which thereducing agent is a material having a reduction potential of higher thanor equal to −0.4 V and lower than or equal to +0.8 V with respect to thepotential of a saturated calomel electrode.

One embodiment of the present invention is a method for forming anelectrode, which includes the steps of forming an active material layerwhich includes a composite particle including an active material andgraphene and a polymer having a poly(vinylidene fluoride) structure overa current collector, and immersing the active material layer in purewater or an aqueous solution to modify the polymer having apoly(vinylidene fluoride) structure by eliminating hydrogen fluoride.

One embodiment of the present invention is a method for forming anelectrode, which includes the steps of forming a current collector byproviding a layer substantially formed of a carbon particle over a metalfoil, forming an active material layer including an active materialparticle and a polymer having a poly(vinylidene fluoride) structure overthe current collector, and immersing the active material layer in purewater or an aqueous solution to eliminate hydrogen fluoride from thepolymer having a poly(vinylidene fluoride) structure.

One embodiment of the present invention is the method for forming anelectrode with any of the above-described structures, in which a polyenestructure or an aromatic ring structure is formed from thepoly(vinylidene fluoride) structure by eliminating hydrogen fluoride.

One embodiment of the present invention is the method for forming anelectrode in any of the above-described structures, in which the aqueoussolution has a pKb of more than pKa−4 and less than pKa+4.

One embodiment of the present invention is the method for forming anelectrode in any of the above-described structures, in which the aqueoussolution has a pH of more than 5 and less than 9.

With one embodiment of the present invention, an electrode having both ahigh electrode strength and favorable electric conductivity can beprovided.

With one embodiment of the present invention, an electrode having bothstrong bonding between a current collector and an active material layerand stably low electrode interface resistance can be provided.

With one embodiment of the present invention, a storage battery having ahigh electrode capacity, high-speed charge and dischargecharacteristics, and improved cycle performance can be provided.

With one embodiment of the present invention, a novel electrode, a novelsecondary battery, or a novel power storage device can be provided. Notethat one embodiment of the present invention is not limited to theseeffects. One embodiment of the present invention may have effects otherthan the above-described effects depending on circumstances. Dependingon circumstances or conditions, one embodiment of the present inventionmight not have any of the above-described effects.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings,

FIGS. 1A to 1C illustrate a storage battery electrode;

FIG. 2 illustrates intermolecular force of graphene oxide in oneembodiment of the present invention;

FIG. 3 illustrates a modified polymer in one embodiment of the presentinvention;

FIGS. 4A and 4B illustrate a coin-type storage battery;

FIG. 5 illustrates a laminated storage battery;

FIGS. 6A and 6B illustrate the laminated storage battery;

FIGS. 7A and 7B illustrate a cylindrical storage battery;

FIG. 8 illustrates examples of an electric device;

FIGS. 9A to 9C illustrate an example of an electric device;

FIGS. 10A and 10B illustrate an example of an electric device;

FIGS. 11A and 11B show analysis results of the particle sizedistribution;

FIG. 12 shows battery characteristics of batteries that use an electrodeof one embodiment of the present invention;

FIGS. 13A to 13D show ToF-SIMS analysis results; and

FIGS. 14A to 14D show ToF-SIMS analysis results.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described below with reference to drawings. However,the embodiments can be implemented in many different modes, and it willbe readily appreciated by those skilled in the art that modes anddetails thereof can be changed in various ways without departing fromthe spirit and scope of the present invention. Thus, the presentinvention should not be interpreted as being limited to the followingdescription of the embodiments.

Embodiment 1

In this embodiment, a method for forming a storage battery electrode ofone embodiment of the present invention will be described with referenceto FIGS. 1A to 1C.

FIG. 1A is a perspective view of a storage battery electrode 100, andFIG. 1B is a longitudinal sectional view of the storage batteryelectrode 100. Although the storage battery electrode 100 in the shapeof a rectangular sheet is illustrated in FIG. 1A, the shape of thestorage battery electrode 100 is not limited thereto and may be anyappropriate shape. An active material layer 102 is formed over only onesurface of a current collector 101 in FIGS. 1A and 1B; however, activematerial layers 102 may be formed so that the current collector 101 issandwiched therebetween. The active material layer 102 does notnecessarily need to be formed over the entire surface of the currentcollector 101 and a region that is not coated, such as a region forconnection to an electrode tab, is provided as appropriate.

There is no particular limitation on the current collector used as apositive-electrode current collector or a negative-electrode currentcollector as long as it has high conductivity without causing asignificant chemical change in a power storage device. For example, thecurrent collector can be formed using a metal such as gold, platinum,iron, nickel, copper, aluminum, titanium, tantalum, or manganese, or analloy thereof (e.g., stainless steel). Alternatively, silicon,neodymium, scandium, molybdenum, or the like may be added to theabove-described metal or the alloy to improve heat resistance.Alternatively, the above-described metal or the alloy that is coatedwith carbon, nickel, titanium, or the like may be used. The currentcollector can each have any of various shapes including a foil-likeshape, a sheet-like shape, a plate-like shape, a net-like shape, acylindrical shape, a coil shape, a punching-metal shape, anexpanded-metal shape, a porous shape, and a shape of non-woven fabric asappropriate. The current collector may be formed to have microirregularities on the surface thereof in order to enhance adhesion tothe active material. The current collector preferably has a thickness ofmore than or equal to 5 μm and less than or equal to 30 μm.

FIG. 1C is a longitudinal sectional view of the active material layer102. The active material layer 102 includes active material particles103, graphene flakes 104 that are graphene serving as a conductiveadditive, and a binder (not illustrated).

The longitudinal section of the active material layer 102 in FIG. 1Cshows substantially uniform dispersion of the sheet-like graphene flakes104 in the active material layer 102. The graphene flakes 104 areschematically shown by heavy lines in FIG. 1C but are actually thinfilms each having a thickness corresponding to the thickness of a singlelayer or a multiple layer of carbon molecules. The plurality of grapheneflakes 104 are formed in such a way as to wrap, coat, or be adhered to aplurality of the active material particles 103, so that the grapheneflakes 104 make surface contact with the active material particles 103.Further, the graphene flakes 104 are also in surface contact with eachother; consequently, the plurality of graphene flakes 104 form athree-dimensional network for electric conduction.

This is because graphene oxide with extremely high dispersibility in apolar solvent is used for formation of graphene. The solvent is removedby volatilization from a dispersion medium including uniformly dispersedgraphene oxide and the graphene oxide is reduced to give graphene;hence, the graphene flakes 104 remaining in the active material layer102 partly overlap with each other and are dispersed such that thesurface contact is made, thereby forming a path for electric conduction.

Unlike a conductive additive in the form of particles such as acetyleneblack, which makes point contact with an active material, graphene iscapable of surface contact with high contact possibility; accordingly,the electric conduction paths among the active material particles 103and the graphene flakes 104 can be formed with a small amount of theconductive additive. Thus, the proportion of the active materialparticles 103 in the active material layer 102 can be increased.Accordingly, the discharge capacity of the storage battery can beincreased.

For the active material particles 103, a material into and from whichcarrier ions such as lithium ions can be inserted and extracted is used.The average diameter or diameter distribution of the active materialparticles can be controlled by crushing, granulation, and classificationby an appropriate means. Although the active material particles 103 areschematically illustrated as spheres in FIG. 1C, the shape of the activematerial particles 103 is not limited to this shape.

The average diameter of a primary particle of the active materialparticles 103 is less than or equal to 500 nm, preferably greater thanor equal to 50 nm and less than or equal to 500 nm. To make surfacecontact with a plurality of the active material particles 103, thegraphene flakes 104 preferably have sides the length of each of which isgreater than or equal to 50 nm and less than or equal to 100 μm,preferably greater than or equal to 800 nm and less than or equal to 20μm.

In the case where the active material particles 103 are positiveelectrode active material particles, a material into and from whichlithium ions can be inserted and extracted can be used; for example, amaterial having an olivine crystal structure, a layered rock-saltcrystal structure, a spinel crystal structure, or a NASICON crystalstructure, or the like can be used.

As the positive electrode active material, a compound such as LiFeO₂,LiCoO₂, LiNiO₂, or LiMn₂O₄, V₂O₅, Cr₂O₅, or MnO₂ can be used, forexample.

Alternatively, an olivine-type lithium-containing complex phosphatewhich can be expressed by a general formula LiMPO₄ (M is one or more ofFe, Mn, Co, and Ni) can be used. Typical examples of LiMPO₄ are lithiummetal phosphate compounds such as LiFePO₄, LiNiPO₄, LiCoPO₄, LiMnPO₄,LiFe_(a)Ni_(b)PO₄, LiFe_(a)Co_(b)PO₄, LiFe_(a)Mn_(b)PO₄,LiNi_(a)Co_(b)PO₄, LiNi_(a)Mn_(b)PO₄ (a+b≤1, 0<a<1, and 0<b<1),LiFe_(c)Ni_(d)Co_(e)PO₄, LiFe_(c)Ni_(d)Mn_(e)PO₄,LiNi_(c)Co_(d)Mn_(e)PO₄ (c+d+e≤1, 0<c<1, 0<d<1, and 0<e<1), andLiFe_(f)Ni_(g)Co_(h)Mn_(i)PO₄ (f+g+h+i≤1, 0<f<0<g<1, 0<h<1, and 0<i<1).

Alternatively, a lithium-containing transition metal silicate such asLi_((2-j))MSiO₄ (general formula) (M is one or more of Fe(II), Mn(II),Co(II), and Ni(II), 0≤j≤2) can be used. Typical examples ofLi_((2-j))MSiO₄ (general formula) are lithium metal silicate compoundssuch as Li_((2-j))FeSiO₄, Li_((2-j))NiSiO₄, Li_((2-j))CoSiO₄,Li_((2-j))MnSiO₄, Li_((2-j))Fe_(k)Ni_(l)SiO₄,Li_((2-j))Fe_(k)Co_(l)SiO₄, Li_((2-j))Fe_(k)Mn_(l)SiO₄,Li_((2-j))Ni_(k)Mn_(l)SiO₄ (k+l≤1, 0<k<1, and 0<l<1),Li_((2-j))Fe_(m)—Ni_(n)Co_(q)SiO₄, Li_((2-j))Fe_(m)Ni_(n)Mn_(q)SiO₄,Li_((2-j))Ni_(m)Co_(n)Mn_(q)SiO₄ (m+n+q≤1, 0<m<1, 0<n<1, and 0<q<1), andLi_((2-j))Fe_(r)Ni_(s)Co_(t)Mn_(u)SiO₄ (r+s+t+u≤1, 0<r<1, 0<s<1, 0<t<1,and 0<u<1).

Still alternatively, a NASICON compound expressed by A_(x)M₂(XO₄)₃(general formula) (A=Li, Na, or Mg, M=Fe, Mn, Ti, V, Nb, or Al, X=S, P,Mo, W, As, or Si) can be used for the active material particles 103.Examples of the NASICON compound are Fe₂(MnO₄)₃, Fe₂(SO₄)₃, andLi₃Fe₂(PO₄)₃. Further alternatively, a compound expressed by Li₂MPO₄F,Li₂MP₂O₇, or Li₅MO₄ (general formula) (M=Fe or Mn), a perovskitefluoride such as NaF₃ or FeF₃, a metal chalcogenide (a sulfide, aselenide, or a telluride) such as TiS₂ or MoS₂, a material with aninverse spinel crystal structure such as LiMVO₄, a vanadium oxide (V₂O₅,V₆O₁₃, LiV₃O₈, or the like), a manganese oxide, an organic sulfurcompound, or the like can be used as the positive electrode activematerial.

In the case where carrier ions are alkali metal ions other than lithiumions, or alkaline-earth metal ions, a compound in which lithium of thelithium compound, the lithium-containing complex phosphate, or thelithium-containing transition metal silicate is replaced by carrier ionsof an alkali metal (e.g., sodium or potassium) or an alkaline-earthmetal (e.g., calcium, strontium, barium, beryllium, or magnesium) may beused as the positive electrode active material.

The average diameter of the positive electrode active material ispreferably, for example, more than or equal to 5 nm and less than orequal to 50 μm.

In the case where a negative electrode active material is used for theactive material particles 103, for example, a carbon-based material, analloy-based material, or the like can be used.

Examples of the carbon-based material include graphite, graphitizingcarbon (soft carbon), non-graphitizing carbon (hard carbon), a carbonnanotube, graphene, carbon black, and the like.

Examples of the graphite include artificial graphite such as meso-carbonmicrobeads (MCMB), coke-based artificial graphite, or pitch-basedartificial graphite and natural graphite such as spherical naturalgraphite.

Graphite has a low potential substantially equal to that of a lithiummetal when lithium ions are inserted into graphite and alithium-graphite intercalation compound is formed. For this reason, alithium ion secondary battery can have a high operating voltage. Inaddition, graphite has advantages of relatively high capacity per unitvolume, small change in volume due to charging and discharging, lowcost, and safety greater than that of a lithium metal.

As the negative electrode active material, a material which enablescharge-discharge reaction by alloying and dealloying reaction with acarrier ion can be used. For example, in the case where carrier ions arelithium ions, a material including at least one of Mg, Ca, Ga, Si, Al,Ge, Sn, Pb, Sb, Bi, Ag, Zn, Cd, As, Hg, In, and the like can be used.Such elements have higher capacity than carbon. In particular, siliconhas a high theoretical capacity of 4200 mAh/g. For this reason, siliconis preferably used as the negative electrode active material. Examplesof the material using such elements include SiO, Mg₂Si, Mg₂Ge, SnO,SnO₂, Mg₂Sn, SnS₂, V₂Sn₃, FeSn₂, CoSn₂, Ni₃Sn₂, Cu₆Sn₅, Ag₃Sn, Ag₃Sb,Ni₂MnSb, CeSb₃, LaSn₃, La₃Co₂Sn₇, CoSb₃, InSb, SbSn, and the like.

Alternatively, for the negative electrode active material, an oxide suchas titanium dioxide (TiO₂), lithium titanium oxide (Li₄Ti₅O₁₂), niobiumpentoxide (Nb₂O₅), tungsten oxide (WO₂), or molybdenum oxide (MoO₂) canbe used.

Still alternatively, for the negative electrode active material,Li_(3-x)M_(x)N (M=Co, Ni, or Cu) with a Li₃N structure, which is anitride including lithium and a transition metal, can be used. Forexample, Li_(2.6)Co_(0.4)N₃ has high charge and discharge capacity (900mAh/g and 1890 mAh/cm³).

In the case of using a nitride including lithium and a transition metal,since lithium ions are included in the negative electrode activematerial, a material which does not include lithium ions, such as V₂O₅or Cr₃O₈, can be used as a positive electrode active material. Note thateven in the case of using a material including lithium ions as apositive electrode active material, the nitride including lithium and atransition metal can be used as the negative electrode active materialby extracting lithium ions included in the positive electrode activematerial in advance.

Alternatively, a material which causes a conversion reaction can be usedas the negative electrode active material; for example, a transitionmetal oxide which does not cause an alloy reaction with lithium, such ascobalt oxide (CoO), nickel oxide (NiO), or iron oxide (FeO), may beused. Other examples of the material which causes a conversion reactioninclude oxides such as Fe₂O₃, CuO, Cu₂O, RuO₂, and Cr₂O₃, sulfides suchas CoS_(0.89), NiS, or CuS, nitrides such as Zn₃N₂, Cu₃N, and Ge₃N₄,phosphides such as NiP₂, FeP₂, and CoP₃, and fluorides such as FeF₃ andSiF₃. Note that any of the fluorides can be used as a positive electrodeactive material because of its high potential.

In this embodiment, graphene is used as the conductive additive.Graphene is obtained by reduction of graphene oxide. Graphene oxide canbe formed by any of a variety of synthesis methods such as a Hummersmethod, a modified Hummers method, or oxidation of a graphite material.

For example, in a Hummers method, graphite such as flake graphite isoxidized to give graphite oxide. The obtained graphite oxide is graphitewhich is oxidized partly and thus to which a functional group such as acarbonyl group, a carboxyl group, or a hydroxyl group is bonded.Accordingly, in the graphite oxide, the crystallinity of the graphite islost and the distance between layers is increased. Therefore, grapheneoxide can be easily obtained by separation of the layers from each otherby ultrasonic treatment or the like.

The length of one side (also referred to as a flake size) of thegraphene oxide is greater than or equal to 50 nm and less than or equalto 100 μm, preferably greater than or equal to 800 nm and less than orequal to 20 μm.

In a storage battery electrode of one embodiment of the presentinvention, this graphene is used as a conductive additive of theelectrode. However, in the case of forming the storage battery electrodeby mixing graphene or grapheme formed by reducing graphene oxide inadvance (RGO is used as an abbreviation of reduced graphene oxide) withan active material and a binder, aggregation of the grapheme or RGOoccurs in the electrode because of the low dispersion property of thegraphene and RGO; therefore, it is difficult to achieve favorablebattery characteristics.

While in the case of using graphene oxide as a raw material of aconductive additive of an electrode, a mixture formed by mixing thegraphene oxide with an active material and a binder in a polar solventis provided over a current collector, and after that, the graphene oxideis reduced by reduction treatment, so that graphene can be formed. Whenan electrode is formed using this method, a graphene network forelectric conduction is formed in an active material layer including anactive material and a binder. Thus, an electrode including a highlyconductive active material layer where active material particles areelectrically connected to each other by graphene can be formed.

This is because graphene oxide used as a raw material of graphene is apolar material having a functional group such as an epoxy group, acarbonyl group, a carboxyl group, or a hydroxyl group as illustrated inFIG. 2. Oxygen in the functional group in graphene oxide is negativelycharged in a polar solvent; hence, graphene oxide flakes do not easilyaggregate. Moreover, graphene oxide strongly interacts with the polarsolvent and the active material used in mixing, and the functional groupsuch as an epoxy group in the graphene oxide interacts with the polarsolvent, which probably prevents aggregation among graphene oxideflakes, resulting in uniform dispersion of graphene oxide in the polarsolvent. Furthermore, graphene oxide flakes are thin; thus, grapheneoxide especially with strong intermolecular force easily bonds to activematerial particles including oxygen. In addition, since graphene oxideflakes do not easily bond to each other, aggregation of active materialparticles including oxygen tends to be suppressed.

When graphene oxide is used as a raw material of a conductive additive,graphene oxide does not function as a conductive additive as it isbecause graphene oxide has high dispersibility in the polar solvent buthas low conductivity. However, because graphene oxide has a stableskeleton derived from graphene, graphene oxide can easily gainconductivity by reduction treatment.

Examples of a method for reducing graphene oxide include reductiontreatment with heating (thermal reduction), electrochemical reductiontreatment performed by applying a potential at which grapheme oxide isreduced to an electrode in an electrolytic solution (electrochemicalreduction), and reduction treatment using a reducing agent (chemicalreduction).

In the case of performing thermal reduction, attention has to be paid sothat decomposition or a change in quality of other materials is notcaused.

In the case of performing electrochemical reduction, attention has to bepaid so as to sufficiently apply voltage evenly to a storage batteryelectrode under the condition where an electrolytic solution does notdissolve materials of the electrode.

In the case of performing chemical reduction, attention has to be paidso that dissolution or a change in quality of an active material or acurrent collector is not caused by a reducing agent or a reducingsolution.

These methods for reducing graphene oxide are described in detail later.

Using the above-described materials, an electrode is formed.

First, graphene oxide and active material particles are mixed, and apolar solvent such as 1-methyl-2-pyrrolidone (NMP) or dimethylformamide(DMF) is added thereto and mixed to prepare a paste mixture. Inparticular, when a material including oxygen is used as the activematerial particles 103, mixing under the state where the active materialparticles and graphene oxide are dispersed uniformly and the state wherethe secondary particle diameter is small, that is, under thedisaggregated state, is possible. Here, “paste” is used to refer to theviscosity at which free flowing does not occur in the stationary state.

The paste mixture is kneaded. Here, “kneading” refers to mixing withhigh viscosity. In kneading, high shearing force is generated andthereby dispersion of an active material or graphene oxide or separationof graphene oxide easily occurs. As a kneading means, an apparatus suchas a planetary mixer or a kneader can be used.

Then, a solvent and a binder are added to the kneaded paste mixture toadjust the viscosity to give a slurry mixture. Here, “slurry” is used torefer to the viscosity at which fluidity is maintained even in thestationary state. As a mixing means of the slurry mixture, any ofvarious kinds of mixers such as a stirring and defoaming machine and adissolver can be used.

Here, a polymer having a poly(vinylidene fluoride) structure is used asthe binder. Examples of the polymer having a poly(vinylidene fluoride)structure include poly(vinylidene fluoride) (PVdF) and a copolymerincluding PVdF (e.g., a copolymer of PVdF and polytetrafluoroethylene(PTFE)). Once PVdF reacts with a base, a chain reaction of eliminatinghydrogen fluoride proceeds. This chain reaction can be suppressed whenusing the copolymer.

Note that a different apparatus may be used depending on the viscosity,or only the viscosity may be gradually changed with a planetarycentrifugal mixer in mixing. Moreover, if stirring is performed at highspeed, sufficient shearing force can be generated from the lowviscosity; thus, uniform mixing is possible by starting the mixing fromthe slurry state without passing through the mixing in the paste state.

The slurry mixture is applied to one or both surfaces of the currentcollector. For the application, a slot die method, a gravure method, ablade method, or a combination of any of them can be used, for example.

Next, the solvent is volatilized by a method such as ventilation dryingor reduced pressure drying, whereby an electrode including the activematerial layer 102 and the current collector 101 is formed. Theventilation drying can be performed with a hot wind with temperatures ofhigher than or equal to 50° C. and lower than or equal to 180° C.Through this step, the polar solvent included in the active materiallayer 102 is volatilized.

Here, the amount of graphene oxide is set to higher than or equal to 0.1wt % and lower than or equal to 10 wt %, preferably higher than or equalto 0.1 wt % and lower than or equal to 5 wt %, further preferably higherthan or equal to 0.2 wt % and lower than or equal to 2 wt %, stillfurther preferably higher than or equal to 0.2 wt % and lower than orequal to 1 wt % with respect to the total weight of the mixture of thegraphene oxide, the active material, the conductive additive, and thebinder. Furthermore, in the active material layer, graphene is includedat higher than or equal to 0.05 wt % and lower than or equal to 5 wt %,preferably higher than or equal to 0.05 wt % and lower than or equal to2.5 wt %, further preferably higher than or equal to 0.1 wt % and lowerthan or equal to 1 wt %, still further preferably higher than or equalto 0.1 wt % and lower than or equal to 0.5 wt % with respect to thetotal weight of the active material layer. This is because the weight ofgraphene is reduced by almost half due to the reduction of the grapheneoxide. The binder is included at higher than or equal to 0.5 wt % andlower than or equal to 20 wt %, preferably higher than or equal 1 wt %and lower than or equal to 10 wt %.

The active material layer 102 is pressed by a compression method such asa roll press method or a flat plate press method so as to have highdensity.

Next, reaction is caused in a solution including a reducing agent (alsoreferred to as a reducing solution). By this reaction, graphene oxideincluded in the active material layer is reduced to form graphene. Notethat oxygen in graphene oxide is not necessarily entirely eliminated andmay partly remain in graphene. When graphene includes oxygen, the ratioof oxygen measured by XPS in graphene is higher than or equal to 2atomic % and lower than or equal to 20 atomic %, preferably higher thanor equal to 3 atomic % and lower than or equal to 15 atomic %. Thisreduction treatment is preferably performed at higher than or equal toroom temperature and lower than or equal to 150° C.

As a solute, a material having a LUMO level of higher than or equal to−5.0 eV and lower than or equal to −3.8 eV, or a reducing agent having areduction potential of higher than or equal to −1.3 V and lower than orequal to +0.8 V (vs. SCE), preferably higher than or equal to −0.4 V andlower than or equal to −0.8 V (vs. SCE) can be used. Examples of thereducing agent are ascorbic acid, hydrazine, dimethyl hydrazine,hydroquinone, sodium boron hydride (NaBH₄), tetra butyl ammonium bromide(TBAB), N,N-diethylhydroxylamine, and a derivative thereof. Particularlywhen a reducing agent with a low reducing ability is used, the influenceon the active material can be suppressed. In addition, decomposition dueto the reaction with the solvent or moisture in the atmospheric air isless likely to occur, so that the selection of the environment for thereduction treatment and the solvent becomes less limited.

A polar solvent can be used as the solvent. Any material can be used forthe polar solvent as long as it can dissolve the reducing agent.Examples of the material of the polar solvent are water, methanol,ethanol, acetone, tetrahydrofuran (THF), DMF, NMP, dimethyl sulfoxide(DMSO), and a mixed solution of any two or more of the above. Inparticular, water is highly advantageous industrially. In addition,water can shorten the reaction time.

The above-described reducing solution including the solvent and thesolute is preferably substantially neutral or weakly basic. In otherwords, the acid dissociation constant pKa and the base dissociationconstant pKb of the reducing solution preferably have the followingrelation: pKb<pKa+4. This facilitates the modification of thepoly(vinylidene fluoride) structure in the polymer. Moreover, therelation pKb>pKa−4 is preferable because excessive modification of thepoly(vinylidene fluoride) structure can be easily suppressed. Furtherpreferably, the relation pKa−2<pKb<pKa+2 is satisfied, in which case themodification of the polymer can be easily controlled. In the case wherethe reducing solution is an aqueous solution, an aqueous solution havingpH of more than 4 and less than 10, preferably more than 5 and less than9, further preferably more than 6 and less than 8, is used. An alkalireagent may be added to the reducing solution.

Then, the electrode is cleaned. As the cleaning liquid, the solvent ofthe reducing solution can be used. The remaining reducing agent can beremoved by the cleaning. In addition, the elimination reaction in RGOcan be promoted by the cleaning. Furthermore, the modification of thepolymer can be caused in this cleaning step as well as in the reductiontreatment step. It can be considered that the promotion of theelimination reaction in RGO is caused by elimination of the reducingagent or a proton that is added and not eliminated. Thus, a slightamount of reagent that facilitates elimination of a proton may be added.Since a certain amount of reducing solution remains on the electrodenormally, the used cleaning liquid includes the reducing agent at aconcentration lower than that of the reducing solution used in thereduction treatment. By repeating the cleaning a plurality of times, theconcentration of the reducing agent included in the used cleaning liquidand on the electrode is lowered. By adding a base into the cleaningliquid, the modification of the polymer can be promoted. In particular,cleaning is preferably performed with pure water because the eliminationreaction in RGO and the modification reaction of the polymer are easilypromoted and an industrially big advantage can be gained.

This modification reaction of the polymer is typically elimination ofhydrogen fluoride, and the polymer having a poly(vinylidene fluoride)structure is modified into a polymer which has a poly(vinylidenefluoride) structure and partly has a polyene structure or an aromaticring structure, which is a cyclized structure. In other words, apoly(vinylidene fluoride) is modified into a partly dehydrofluorinatedpoly(vinylidene fluoride) which has a polyene structure or an aromaticring structure in its main chain. A molecular structure example of themodified polymer is illustrated in FIG. 3. The formed polyene structureor the aromatic ring structure, which is a cyclized structure, issandwiched between the unmodified poly(vinylidene fluoride) structures.Note that the polyene structure herein refers to a partial structurerepresented by (CH)_(n) (n>3). The aromatic ring structure is an arylgroup including a polycyclic aromatic hydrocarbon group, and includes aphenyl group, a biphenyl group, or a naphthyl group. The poly(vinylidenefluoride) structure is a partial structure represented by (CF₂CH₂)_(n).Although more than one hundred monomers are included in the polymer, thesame monomer structure does not necessarily continue in the case wherethe polymer is a random copolymer. Furthermore, the modification rate,that is, the sum of the formed aromatic ring structures and the formedpolyene structures is less than 5% in carbon atomic ratio with respectto the poly(vinylidene fluoride) structures.

Then, the cleaned electrode is heated. The heating of the electrode ispreferably performed under reduced pressure. This heating step ispreferably performed at, for example, a temperature of higher than orequal to 50° C. and lower than or equal to 200° C. in vacuum for morethan or equal to 1 hour and less than or equal to 48 hours. By thisheating, the polar solvent or moisture existing in the active materiallayer 102 can be removed. Furthermore, reduction of RGO can be promoted.Through this process, another thermal condensation process, or the like,the electrode strength can be improved.

The obtained electrode is pressed by a compression method such as a rollpress method or a flat plate press method so as to have high density.The pressing may be performed either or both of before and after thereduction treatment. The obtained electrode is shaped to a predeterminedsize, and a storage battery electrode is formed. Since the entry ofmoisture into the storage battery is particularly a significant problem,it is preferable to press the storage battery while moisture is removedin a reduced-pressure atmosphere. However, in terms of processing, it ispreferable to press the storage battery in an air atmosphere. Thus,after the reduction treatment, the electrode is heated under a reducedpressure, pressed in an air atmosphere, and then shaped to apredetermined size. In addition, the obtained electrode is preferablyfurther heated under a reduced pressure so that moisture can besufficiently removed.

Since the bonding between the modified polymer and RGO is improved, theelectrode strength can be improved. In addition, since RGO is evenlydispersed in the electrode together with the active material particles,a voltage can be applied evenly to the entire electrode. This suppressespolarization of part of the active material particles in the electrode,so that a capacity is increased. Furthermore, since the preferentialbattery reaction in part of the particles is suppressed, cycleperformance is increased. Moreover, since the active material particlesare favorably bonded to each other with the binder and RGO, for example,even when interface resistance of the active material is increased owingto the decomposition of the electrolytic solution, the influence islittle. This means a high ability of suppressing the change in internalresistance of the battery, leading to improvements in cycle performanceand storage characteristics.

Through the above-described steps, the storage battery electrode 100which includes the polymer having the modified poly(vinylidene fluoride)structure, graphene, and the active material particles can be formed.

One embodiment of the present invention can be used for various powerstorage devices. Examples of the power storage devices include abattery, a primary battery, a secondary battery, a lithium-ion secondarybattery (including a lithium-ion polymer secondary battery), and alithium air battery. The examples of the power storage devices alsoinclude a capacitor.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Embodiment 2

In this embodiment, a method for forming a storage battery electrodewith improved interface bonding strength between an active materiallayer and a current collector will be described in detail.

First, a slurry mixture which includes active material particles, apolymer having a poly(vinylidene fluoride) structure, and a conductiveadditive is prepared. Here, the conductive additive is not particularlylimited, and carbon black, especially carbon black having a higher orderstructure such as acetylene black or Ketjen Black (registeredtrademark), can be used. Alternatively, the above-described graphene orthe like can be used.

A metal foil covered with a layer substantially formed of carbonparticles is used as the current collector. The kind of the metal foilis as described in Embodiment 1, and a feature of this embodiment isthat the metal foil is covered with a layer substantially formed ofcarbon particles. The layer substantially formed of carbon particlesdoes not necessarily cover the entire current collector, and rathertends to be in an island shape covering part of the current collector,in which case part of the current collector metal is exposed. The layersubstantially formed of carbon particles may include carbon particleswith almost the same size as that of graphite or may include only carbonblack having less than or equal to the submicron size. The layersubstantially formed of carbon particles can be formed by preparing adispersion liquid in which carbon particles are dispersed in a solventand coating the current collector with the dispersion liquid. Thedispersion liquid in which carbon particles are dispersed includes anorganic polymer at 1 wt % or lower or does not include an organicpolymer at all. As the dispersion liquid in which carbon particles aredispersed, an aqueous solution in which carbon black is dispersed can beused. As a dispersion means, a method for adding a base such as ammoniais known, for example. As the carbon particles, carbon black ispreferably used. The layer formed of carbon particles preferably has athickness of 2 μm or less.

To the current collector covered with the layer substantially formed ofcarbon particles, the slurry mixture including the active materialparticles is applied to form an active material layer. As for theapplication means and the heating means for volatilizing the solvent,any of the methods described in Embodiment 1 can be employed.

The obtained electrode is immersed in the polar solvent, so that thepolymer having a poly(vinylidene fluoride) structure in the activematerial layer can be modified. This modification is typicallyelimination of hydrogen fluoride, and the polymer having apoly(vinylidene fluoride) structure is modified into a polymer which hasa poly(vinylidene fluoride) structure and partly has a polyene structureor an aromatic ring structure, which is a cyclized structure. In thecase of using a slurry mixture including graphene oxide in order to usegraphene as the conductive additive, the reduction step of grapheneoxide and the modification step of the poly(vinylidene fluoride)structure can be performed in one step. As the polar solvent, water oralcohols such as ethanol can be used. The liquid in which the electrodeis immersed may include a pH adjuster. In this case, however, the pHneed not necessarily be changed. This is because the hydrofluoric acideliminated by modification is an acid. In the case of modifying thepolymer in parallel with another treatment, the pH (or pKa) may bechanged in accordance with the another treatment. The modification ofthe polymer proceeds more when the liquid in which the electrode isimmersed is basic, but can be performed depending on the time andtemperature even when the liquid in which the electrode is immersed isneutral or weakly acidic. However, if the modification of the polymerproceeds excessively, gelling of the polymer or a reduction of thebinder ability are caused; accordingly, it is preferable that only partof the polymer be modified. In this case, the formed polyene structureor the aromatic ring structure, which is a cyclized structure, issandwiched between the unmodified poly(vinylidene fluoride) structures.As described above, the polyene structure herein refers to a partialstructure represented by (CH)_(n) (n>3). The aromatic ring structure isan aryl group including a polycyclic aromatic hydrocarbon group, andincludes a phenyl group, a biphenyl group, or a naphthyl group. Thepoly(vinylidene fluoride) structure is a partial structure representedby (CF₂CH₂)_(n). Although more than one hundred monomers are included inthe polymer, the same monomer structure does not necessarily continue inthe case where the polymer is a random copolymer.

After the modification treatment, the electrode is heated to volatilizethe included solvent. Cleaning may be performed after the modificationtreatment, as appropriate. The heating may be performed under anatmospheric pressure or a reduced pressure. The heating under anatmospheric pressure and the heating under a reduced pressure may beperformed at different temperatures in combination.

The obtained electrode may be pressed by a compression method such as aroll press method or a flat plate press method so as to have highdensity. The pressing may be performed either or both of before andafter the modification treatment. Since the entry of moisture into thestorage battery is particularly a significant problem, it is preferableto press the storage battery while moisture is removed in areduced-pressure atmosphere. However, in terms of processing, it ispreferable to press the storage battery in an air atmosphere. Thus, itis preferable that after the modification treatment, the electrode beheated and pressed and that water and the solvent be elaboratelyvolatilized.

Lastly, the electrode is shaped to a predetermined size, and a storagebattery electrode is formed.

In the vicinity of the interface of the current collector, the layersubstantially formed of carbon particles (also referred to as a“covering layer”) is sandwiched between the metal foil and the activematerial layer. An oxide layer is sometimes formed on the surface of themetal foil, and the oxide layer generates interface resistance. Byforming the covering layer on the surface of the metal foil, the surfaceof the metal foil in contact with the covering layer is reduced, so thatformation of the oxide layer is suppressed and the interface resistanceis hardly generated.

The materials which are used as carbon particles included in thecovering layer include few materials that generate favorableintermolecular force. Thus, for example, by mixing a polymer into thecovering layer, the application conditions of a mixed liquid for formingthe covering layer are improved. Furthermore, the interface strength ofthe electrode can be improved. However, in the case of using thisembodiment, a structure having the same conjugated π bond as the carbonparticles included in the covering layer is formed in the polymerincluded in the active material layer, so that the bonding strengthbetween the active material layer and the covering layer is improved.Furthermore, since the covering layer and the metal foil are bothconductors, the intermolecular force is relatively favorable. For theabove-described reasons, in this embodiment, an electrode with improvedinterface strength can be obtained without mixing a polymer into thecovering layer.

Compared with the case where the current collector in which a metal foilis covered with the covering layer into which a polymer is mixed is usedand the polymer modification treatment is not performed, this embodimentcan improve the bonding strength at the interface between the metal foiland the covering layer and provide an electrode in which separation inthe electrode is not easily caused.

This can be explained as follows. In the case of using a covering layerin which carbon particles and a polymer are mixed, the intermolecularforce between the polymer in the covering layer and the polymer having apoly(vinylidene fluoride) structure in the active material layer islarge, and thus the interface strength between the covering layer andthe active material layer is higher than that of this embodiment.However, the bonding strength between the covering layer and the metalfoil is weaker than that of this embodiment because it depends mainly onthe bonding strength between the oxide layer and the polymer. In otherwords, the difference between the bonding strength between the coveringlayer and the metal foil and the bonding strength between the coveringlayer and the active material layer appears more significant than thatof this embodiment. Therefore, when force that causes separation betweenthe current collector and the active material layer is applied to theelectrode, the separation force is concentrated between the coveringlayer and the active material layer.

As compared to the above-mentioned case, this embodiment can improve theelectrode strength. Note that although the intermolecular force workingbetween the carbon particles included in the covering layer and thepolymer included in the active material layer is described as the factorfor increasing the bonding strength between the covering layer and theactive material layer in the above consideration, bonding can beconsidered as the factor, in which case the factor is polymerizationbetween the carbon particles included in the covering layer and themodified polymer included in the active material layer.

Through the above-described steps, a storage battery electrode whichincludes a metal foil, a layer substantially formed of carbon particles(covering layer), and an active material layer including the polymerhaving the modified poly(vinylidene fluoride) structure can be formed.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Embodiment 3

In this embodiment, another method for forming the storage batteryelectrode described in Embodiment 1 will be described in detail.

First, graphene oxide, active material particles, and a polar solventare mixed to prepare a paste mixture. Then, the paste mixture is kneaded(mixing at high viscosity). Since mixing at high viscosity tends togenerate high shearing force, separation of graphene oxide and uniformdispersion of graphene oxide and the active material particles can beachieved. In particular, when a material including oxygen is used as theactive material particles 103, mixing under the state where the activematerial particles and graphene oxide are dispersed uniformly and thestate where the secondary particle diameter is small, that is, under thedisaggregated state, is possible.

The paste mixture is kneaded. Here, “kneading” refers to mixing withhigh viscosity. In kneading, high shearing force is generated andthereby dispersion of an active material or graphene oxide or separationof graphene oxide easily occurs. As a kneading means, an apparatus suchas a planetary mixer or a kneader can be used.

A solvent may be further added into the dispersion mixture. In general,a lower concentration causes less reaggregation, leading to a smallersecondary particle diameter.

Next, reaction of the obtained mixture is caused in a solution includinga reducing agent. By this reaction, graphene oxide included in themixture is reduced to form graphene. Note that oxygen in graphene oxideis not necessarily entirely eliminated and may partly remain ingraphene. When graphene includes oxygen, the ratio of oxygen measured byXPS in graphene is higher than or equal to 1 atomic % and lower than orequal to 20 atomic %, preferably higher than or equal to 2 atomic % andlower than or equal to 15 atomic %. This reduction treatment ispreferably performed at higher than or equal to room temperature andlower than or equal to 150° C. This reduction may be performed after themixture is heated, or a reducing solution may be added to a slurry orpaste mixture. In the case where the reduction is performed after themixture is heated, the heating step causes aggregation, but thenaggregated particles which are extremely small (disaggregated) activematerial secondary particles to which graphene oxide is attached areformed. By selecting the mixing conditions, for example, primaryparticles each of which has a diameter of 100 nm or less and to each ofwhich graphene oxide is attached can be obtained. Thus, the aggregatedparticles obtained by heating can be primary particles each of which isin contact with graphene oxide. These aggregated particles have aconduction path that enables a very efficient battery reaction and canproduce favorable battery characteristics. The degree of difficulty ofapplying particles depends on the diameter of secondary particles, andapplication of submicron microparticles is difficult. By usingaggregated particles, such a difficulty can be eliminated. By thistreatment, graphene oxide becomes RGO.

As a solute, a material having a LUMO level of higher than or equal to−5.0 eV and lower than or equal to −3.8 eV, or a reducing agent having areduction potential of higher than or equal to −1.3 V and lower than orequal to +0.8 V (vs. SCE), preferably higher than or equal to −0.4 V andlower than or equal to +0.8 V (vs. SCE) can be used. Examples of thereducing agent are ascorbic acid, hydrazine, dimethyl hydrazine,hydroquinone, sodium boron hydride (NaBH₄), tetra butyl ammonium bromide(TBAB), N,N-diethylhydroxylamine, and a derivative thereof. Particularlywhen a reducing agent with a low reducing ability is used, the influenceon the active material can be suppressed. In addition, decomposition dueto the reaction with the solvent or moisture in the atmospheric air isless likely to occur, so that the selection of the environment for thereduction treatment and the solvent becomes less limited.

A polar solvent can be used as the solvent. Any material can be used forthe polar solvent as long as it can dissolve the reducing agent.Examples of the material of the polar solvent are water, methanol,ethanol, acetone, THF, DMF, NMP, DMSO, and a mixed solution of any twoor more of the above. In particular, water is highly advantageousindustrially. In addition, water can shorten the reaction time.

Then, the obtained particles are collected. At the same time, cleaningmay be performed. As the cleaning liquid, the solvent of the reducingsolution can be used. The remaining reducing agent can be removed by thecleaning. In addition, the elimination reaction in RGO can be promotedby the cleaning. In particular cleaning is preferably performed withpure water because the elimination reaction in RGO is easily promotedand an industrially big advantage can be gained. For the collection,centrifugation, filtration, or the like can be employed.

The collected particles are heated, so that the remaining solvent isremoved. The heating may also serve as thermal reduction of RGO; in thiscase, the heating can be rephrased as heat treatment. The heat treatmentcan be performed in a vacuum atmosphere, an air atmosphere, or an inertatmosphere. An evaporator or the like may also be used. A plurality ofheating conditions may be combined. The heat treatment can be performedat temperatures ranging from room temperature to approximately 800° C.Much higher temperatures are preferable because oxygen in RGO is furthereliminated and electric conductivity is improved, and the heat treatmentcan be performed in a temperature range that does not causedecomposition of the active material.

A binder and a solvent are added to the obtained mixed particles of theactive material and RGO to adjust the viscosity to give a slurrymixture. As a mixing means of the slurry mixture, any of various kindsof mixers such as a stirring and defoaming machine and a dissolver canbe used.

Here, a polymer having a poly(vinylidene fluoride) structure is used asthe binder. Examples of the polymer having a poly(vinylidene fluoride)structure include PVdF and a copolymer including PVdF (e.g., a copolymerof PVdF and PTFE). Once PVdF reacts with a base, a chain reaction ofeliminating hydrogen fluoride proceeds. This chain reaction can besuppressed by using a copolymer.

Moisture is added to the slurry mixture. This treatment causesmodification of the poly(vinylidene fluoride) structure, that is,elimination of hydrogen fluoride. In other words, part of thepoly(vinylidene fluoride) structure becomes a polyene structure or anaromatic ring structure through cyclization. As the moisture, moistureincluded in the solvent used for slurry formation moisture in theworking atmosphere in or after the mixing may be utilized, instead offurther adding moisture for this treatment. If the modification of PVdFproceeds excessively, gelling of the polymer or a reduction of thebinder function are caused; accordingly, it is preferable that only partof the PVdF be modified. In other words, it is preferable thatmodification occurs in only part of a sequence of poly(vinylidenefluoride) structure. In this case, the formed polyene structure or thearomatic ring structure is sandwiched between the poly(vinylidenefluoride) structures.

After the binder is fully modified, the slurry mixture is applied to oneor both surfaces of the current collector. For the application, a slotdie method, a gravure method, a blade method, or a combination of any ofthem can be used, for example.

Next, the solvent included in the mixture is volatilized by a methodsuch as ventilation drying or reduced pressure drying, whereby anelectrode including the active material layer 102 and the currentcollector is formed. The ventilation drying can be performed with a hotwind with temperatures of higher than or equal to 50° C. and lower thanor equal to 180° C. Through this step, the polar solvent included in theactive material layer 102 is volatilized.

Note that the amount of graphene oxide that is a raw material ispreferably set to higher than or equal to 0.1 wt % and lower than orequal to 10 wt %, preferably higher than or equal to 0.1 wt % and lowerthan or equal to 5 wt %, further preferably higher than or equal to 0.2wt % and lower than or equal to 2 wt %, still further preferably higherthan or equal to 0.2 wt % and lower than or equal to 1 wt % with respectto the total weight of the mixture of the graphene oxide, the positiveelectrode active material, the conductive additive, and the binder.Furthermore, in the electrode, graphene is included at higher than orequal to 0.05 wt % and lower than or equal to 5 wt %, preferably higherthan or equal to 0.05 wt % and lower than or equal to 2.5 wt %, furtherpreferably higher than or equal to 0.1 wt % and lower than or equal to 1wt %, still further preferably higher than or equal to 0.1 wt % andlower than or equal to 0.5 wt % with respect to the total weight of theactive material layer. This is because the weight of graphene is reducedby almost half due to the reduction of the graphene oxide. The binder isincluded at higher than or equal to 0.5 wt % and lower than or equal to20 wt %, preferably higher than or equal to 1 wt % and lower than orequal to 10 wt %.

The active material layer is preferably pressed by a compression methodsuch as a roll press method or a flat plate press method so as to havehigh density.

Through the above-described steps, the storage battery electrode 100which includes the polymer having the modified poly(vinylidene fluoride)structure, graphene, and the active material particles can be formed.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Embodiment 4

In this embodiment, the structure of a storage battery including astorage battery electrode formed by the formation method described inEmbodiment 1 will be described with reference to FIGS. 4A and 4B, FIG.5, FIGS. 6A and 6B, and FIGS. 7A and 7B.

(Coin-Type Storage Battery)

FIG. 4A is an external view of a coin-type (single-layer flat type)storage battery, and FIG. 4B is a cross-sectional view thereof.

In a coin-type storage battery 300, a positive electrode can 301doubling as a positive electrode terminal and a negative electrode can302 doubling as a negative electrode terminal are insulated from eachother and sealed by a gasket 303 made of polypropylene or the like. Apositive electrode 304 includes a positive electrode current collector305 and a positive electrode active material layer 306 provided incontact with the positive electrode current collector 305. A negativeelectrode 307 includes a negative electrode current collector 308 and anegative electrode active material layer 309 provided in contact withthe negative electrode current collector 308. A separator 310 and anelectrolytic solution (not illustrated) are provided between thepositive electrode active material layer 306 and the negative electrodeactive material layer 309.

As the positive electrode 304 and the negative electrode 307, storagebattery electrodes formed by the method for forming a storage batteryelectrode of one embodiment of the present invention, which is describedin Embodiment 1, can be used.

As the separator 310, an insulator including pores, such as cellulose(paper), polyethylene, or polypropylene can be used.

As an electrolyte, a solid electrolyte, an electrolytic solutioncontaining a supporting electrolyte, or a gel electrolyte obtained bygelation of part of an electrolytic solution can be used.

As a supporting electrolyte, a material which contains carrier ions canbe used. Typical examples of the supporting electrolyte are lithiumsalts such as LiPF₆, LiClO₄, LiAsF₆, LiBF₄, LiCF₃SO₃, Li(CF₃SO₂)₂N, andLi(C₂F₅SO₂)₂N. One of these electrolytes may be used alone or two ormore of them may be used in an appropriate combination and in anappropriate ratio.

Note that when carrier ions are alkali metal ions other than lithiumions or alkaline-earth metal ions, instead of lithium in the abovelithium salts, an alkali metal (e.g., sodium or potassium), analkaline-earth metal (e.g., calcium, strontium, barium, beryllium, ormagnesium) may be used for the electrolyte.

As a solvent of the electrolytic solution, a material in which carrierions can move can be used. As the solvent of the electrolytic solution,an aprotic organic solvent is preferably used. Typical examples ofaprotic organic solvents include ethylene carbonate (EC), propylenecarbonate, dimethyl carbonate, diethyl carbonate (DEC), γ-butyrolactone,acetonitrile, dimethoxyethane, THF, and the like, and one or more ofthese materials can be used. When a gelled high-molecular material isused as the solvent of the electrolytic solution, safety against liquidleakage and the like is improved. Further, the storage battery can bethinner and more lightweight. Typical examples of gelled high-molecularmaterials include a silicone gel, an acrylic gel, an acrylonitrile gel,polyethylene oxide-based gell, polypropylene oxide-based gell, afluorine-based polymer gell, and the like. Alternatively, the use of oneor more of ionic liquids (particularly, room temperature molten salts)which have features of non-flammability and non-volatility as a solventof the electrolytic solution can prevent the storage battery fromexploding or catching fire even when the storage battery internallyshorts out or the internal temperature increases owing to overchargingor the like. An ionic liquid contains a cation and an anion. Examples ofan organic cation included in an ionic liquid include aliphatic oniumcations such as a quaternary ammonium cation, a tertiary sulfoniumcation, and a quaternary phosphonium cation, and aromatic cations suchas an imidazolium cation and a pyridinium cation. Examples of the anionused for the electrolytic solution include a monovalent amide-basedanion, a monovalent methide-based anion, a fluorosulfonate anion, aperfluoroalkylsulfonate anion, tetrafluoroborate, perfluoroalkylborate,hexafluorophosphate, and perfluoroalkylphosphate.

Instead of the electrolytic solution, a solid electrolyte including aninorganic material such as a sulfide-based inorganic material or anoxide-based inorganic material, or a solid electrolyte including ahigh-molecular material such as a polyethylene oxide (PEO)-basedhigh-molecular material may alternatively be used. When the solidelectrolyte is used, a separator or a spacer is not necessary. Further,the battery can be entirely solidified; therefore, there is nopossibility of liquid leakage and thus the safety of the battery isdramatically increased.

For the positive electrode can 301 and the negative electrode can 302, ametal having a corrosion-resistant property to a liquid such as anelectrolytic solution in charging and discharging a secondary battery,such as nickel, aluminum, or titanium; an alloy of any of the metals; analloy containing any of the metals and another metal (e.g., stainlesssteel); a stack of any of the metals; a stack including any of themetals and any of the alloys (e.g., a stack of stainless steel andaluminum); or a stack including any of the metals and another metal(e.g., a stack of nickel, iron, and nickel) can be used. The positiveelectrode can 301 and the negative electrode can 302 are electricallyconnected to the positive electrode 304 and the negative electrode 307,respectively.

The negative electrode 307, the positive electrode 304, and theseparator 310 are immersed in the electrolyte. Then, as illustrated inFIG. 4B, the positive electrode 304, the separator 310, the negativeelectrode 307, and the negative electrode can 302 are stacked in thisorder with the positive electrode can 301 positioned at the bottom, andthe positive electrode can 301 and the negative electrode can 302 aresubjected to pressure bonding with the gasket 303 interposedtherebetween. In such a manner, the coin-type storage battery 300 can bemanufactured.

(Laminated Storage Battery)

FIG. 5 is an external view of a laminated storage battery 500. FIGS. 6Aand 6B are cross-sectional views along dashed-dotted lines A1-A2 andB1-B2, respectively, in FIG. 5. The laminated storage battery 500 isformed with a positive electrode 503 including a positive electrodecurrent collector 501 and a positive electrode active material layer502, a negative electrode 506 including a negative electrode currentcollector 504 and a negative electrode active material layer 505, aseparator 507, an electrolytic solution 508, and an exterior body 509.The separator 507 is provided between the positive electrode 503 and thenegative electrode 506 in the exterior body 509. The electrolyticsolution 508 is provided in the region surrounded by the exterior body509.

In the laminated storage battery 500 illustrated in FIG. 5, the positiveelectrode current collector 501 and the negative electrode currentcollector 504 also function as terminals for electrical contact with anexternal portion. For this reason, each of the positive electrodecurrent collector 501 and the negative electrode current collector 504is provided so as to be partly exposed to the outside of the exteriorbody 509.

As the exterior body 509 in the laminated storage battery 500, forexample, a laminate film having a three-layer structure where a highlyflexible metal thin film of aluminum, stainless steel, copper, nickel,or the like is provided over a film formed of a material such aspolyethylene, polypropylene, polycarbonate, ionomer, or polyamide, andan insulating synthetic resin film of a polyamide resin, a polyesterresin, or the like is provided as the outer surface of the exterior bodyover the metal thin film can be used. With such a three-layer structure,permeation of an electrolytic solution and a gas can be blocked and aninsulating property and resistance to the electrolytic solution can beobtained.

(Cylindrical Storage Battery)

Next, an example of a cylindrical storage battery will be described withreference to FIGS. 7A and 7B. As illustrated in FIG. 7A, a cylindricalstorage battery 600 includes a positive electrode cap (battery cap) 601on the top surface and a battery can (outer can) 602 on the side surfaceand bottom surface. The positive electrode cap 601 and the battery can602 are insulated from each other by a gasket (insulating gasket) 610.

FIG. 7B is a diagram schematically illustrating a cross section of thecylindrical storage battery. Inside the battery can 602 having a hollowcylindrical shape, a battery element in which a strip-like positiveelectrode 604 and a strip-like negative electrode 606 are wound with astripe-like separator 605 interposed therebetween is provided. Althoughnot illustrated, the battery element is wound around a center pin. Oneend of the battery can 602 is closed and the other end thereof is open.For the battery can 602, a metal having a corrosion-resistant propertyto a liquid such as an electrolytic solution in charging and discharginga secondary battery, such as nickel, aluminum, or titanium; an alloy ofany of the metals; an alloy containing any of the metals and anothermetal (e.g., stainless steel); a stack of any of the metals; a stackincluding any of the metals and any of the alloys (e.g., a stack ofstainless steel and aluminum); or a stack including any of the metalsand another metal (e.g., a stack of nickel, iron, and nickel) can beused. Inside the battery can 602, the battery element in which thepositive electrode, the negative electrode, and the separator are woundis interposed between a pair of insulating plates 608 and 609 which faceeach other. Further, a nonaqueous electrolytic solution (notillustrated) is injected inside the battery can 602 provided with thebattery element. As the nonaqueous electrolytic solution, a nonaqueouselectrolytic solution which is similar to those of the above coin-typestorage battery and the laminated storage battery can be used.

The positive electrode 604 and the negative electrode 606 can be formedin a manner similar to that of the positive electrode and the negativeelectrode of the coin-type storage battery described above except thatactive materials are formed on both sides of the current collectorsowing to the winding of the positive electrode and the negativeelectrode of the cylindrical storage battery. A positive electrodeterminal (positive electrode current collecting lead) 603 is connectedto the positive electrode 604, and a negative electrode terminal(negative electrode current collecting lead) 607 is connected to thenegative electrode 606. Both the positive electrode terminal 603 and thenegative electrode terminal 607 can be formed using a metal materialsuch as aluminum. The positive electrode terminal 603 and the negativeelectrode terminal 607 are resistance-welded to a safety valve mechanism612 and the bottom of the battery can 602, respectively. The safetyvalve mechanism 612 is electrically connected to the positive electrodecap 601 through a positive temperature coefficient (PTC) element 611.The safety valve mechanism 612 cuts off electrical connection betweenthe positive electrode cap 601 and the positive electrode 604 when theinternal pressure of the battery exceeds a predetermined thresholdvalue. Further, the PTC element 611, which serves as a thermallysensitive resistor whose resistance increases as temperature rises,limits the amount of current by increasing the resistance, in order toprevent abnormal heat generation. Note that barium titanate(BaTiO₃)-based semiconductor ceramic or the like can be used for the PTCelement.

Note that in this embodiment, the coin-type storage battery, thelaminated storage battery, and the cylindrical storage battery are givenas examples of the storage battery; however, any of storage batterieswith a variety of shapes, such as a sealed storage battery and asquare-type storage battery, can be used. Further, a structure in whicha plurality of positive electrodes, a plurality of negative electrodes,and a plurality of separators are stacked or wound may be employed.

As the positive electrodes and the negative electrodes of the storagebattery 300, the storage battery 500, and the storage battery 600, whichare described in this embodiment, electrodes formed by the method forforming a storage battery electrode of one embodiment of the presentinvention are used. Thus, the discharge capacity of the storagebatteries 300, 500, and 600 can be increased.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Embodiment 5

A storage battery including the storage battery electrode of oneembodiment of the present invention can be used for power supplies of avariety of electric devices driven by electric power.

Specific examples of electric devices each utilizing a storage batteryincluding the storage battery electrode of one embodiment of the presentinvention are as follows: display devices of televisions, monitors, andthe like, lighting devices, desktop personal computers and laptoppersonal computers, word processors, image reproduction devices whichreproduce still images and moving images stored in recording media suchas digital versatile discs (DVDs), portable CD players, portable radios,tape recorders, headphone stereos, stereos, table clocks, wall clocks,cordless phone handsets, transceivers, mobile phones, car phones,portable game machines, calculators, portable information terminals,electronic notebooks, e-book readers, electronic translators, audioinput devices, video cameras, digital still cameras, toys, electricshavers, high-frequency heating appliances such as microwave ovens,electric rice cookers, electric washing machines, electric vacuumcleaners, water heaters, electric fans, hair dryers, air-conditioningsystems such as air conditioners, humidifiers, and dehumidifiers,dishwashers, dish dryers, clothes dryers, futon dryers, electricrefrigerators, electric freezers, electric refrigerator-freezers,freezers for preserving DNA, flashlights, electrical tools such as achain saw, smoke detectors, and medical equipment such as dialyzers.Further, industrial equipment such as guide lights, traffic lights,conveyor belts, elevators, escalators, industrial robots, power storagesystems, and power storage devices for leveling the amount of powersupply and smart grid can be given. In addition, moving objects drivenby electric motors using electric power from the storage batteries arealso included in the category of electric devices. Examples of themoving objects are electric vehicles (EV), hybrid electric vehicles(HEV) which include both an internal-combustion engine and a motor,plug-in hybrid electric vehicles (PHEV), tracked vehicles in whichcaterpillar tracks are substituted for wheels of these vehicles,motorized bicycles including motor-assisted bicycles, motorcycles,electric wheelchairs, golf carts, boats, ships, submarines, helicopters,aircrafts, rockets, artificial satellites, space probes, planetaryprobes, and spacecrafts.

In the electric devices, the storage battery including the storagebattery electrode of one embodiment of the present invention can be usedas a main power supply for supplying enough electric power for almostthe whole power consumption. Alternatively, in the electric devices, thestorage battery including the storage battery electrode of oneembodiment of the present invention can be used as an uninterruptiblepower supply which can supply electric power to the electric deviceswhen the supply of electric power from the main power supply or acommercial power supply is stopped. Still alternatively, in the electricdevices, the storage battery including the storage battery electrode ofone embodiment of the present invention can be used as an auxiliarypower supply for supplying electric power to the electric devices at thesame time as the power supply from the main power supply or a commercialpower supply.

FIG. 8 illustrates specific structures of the electric devices. In FIG.8, a display device 700 is an example of an electric device including astorage battery 704 including the storage battery electrode of oneembodiment of the present invention. Specifically, the display device700 corresponds to a display device for TV broadcast reception andincludes a housing 701, a display portion 702, speaker portions 703, andthe storage battery 704. The storage battery 704 including the storagebattery electrode of one embodiment of the present invention is providedin the housing 701. The display device 700 can receive electric powerfrom a commercial power supply. Alternatively, the display device 700can use electric power stored in the storage battery 704. Thus, thedisplay device 700 can be operated with the use of the storage battery704 including the storage battery electrode of one embodiment of thepresent invention as an uninterruptible power supply even when electricpower cannot be supplied from a commercial power supply due to powerfailure or the like.

A semiconductor display device such as a liquid crystal display device,a light-emitting device in which a light-emitting element such as anorganic EL element is provided in each pixel, an electrophoresis displaydevice, a digital micromirror device (DMD), a plasma display panel(PDP), or a field emission display (FED) can be used for the displayportion 702.

Note that the display device includes, in its category, all ofinformation display devices for personal computers, advertisementdisplays, and the like besides for TV broadcast reception.

In FIG. 8, an installation lighting device 710 is an example of anelectric device including a storage battery 713 including the storagebattery electrode of one embodiment of the present invention.Specifically, the lighting device 710 includes a housing 711, a lightsource 712, and the storage battery 713. Although FIG. 8 illustrates thecase where the storage battery 713 is provided in a ceiling 714 on whichthe housing 711 and the light source 712 are installed, the storagebattery 713 may be provided in the housing 711. The lighting device 710can receive electric power from a commercial power supply.Alternatively, the lighting device 710 can use electric power stored inthe storage battery 713. Thus, the lighting device 710 can be operatedwith the use of the storage battery 713 including the storage batteryelectrode of one embodiment of the present invention as anuninterruptible power supply even when electric power cannot be suppliedfrom a commercial power supply due to power failure or the like.

Note that although the installation lighting device 710 provided in theceiling 714 is illustrated in FIG. 8 as an example, the storage batteryincluding the storage battery electrode of one embodiment of the presentinvention can be used in an installation lighting device provided in,for example, a wall 715, a floor 716, a window 717, or the like otherthan the ceiling 714. Alternatively, the storage battery including thestorage battery electrode of one embodiment of the present invention canbe used in a tabletop lighting device or the like.

As the light source 712, an artificial light source which emits lightartificially by using electric power can be used. Specifically, anincandescent lamp, a discharge lamp such as a fluorescent lamp, andlight-emitting elements such as an LED and an organic EL element aregiven as examples of the artificial light source.

In FIG. 8, an air conditioner including an indoor unit 720 and anoutdoor unit 724 is an example of an electric device including a storagebattery 723 including the storage battery electrode of one embodiment ofthe present invention. Specifically, the indoor unit 720 includes ahousing 721, an air outlet 722, and the storage battery 723. AlthoughFIG. 8 illustrates the case where the storage battery 723 is provided inthe indoor unit 720, the storage battery 723 may be provided in theoutdoor unit 724. Alternatively, the storage batteries 723 may beprovided in both the indoor unit 720 and the outdoor unit 724. The airconditioner can receive electric power from a commercial power supply.Alternatively, the air conditioner can use electric power stored in thestorage battery 723. Particularly in the case where the storagebatteries 723 are provided in both the indoor unit 720 and the outdoorunit 724, the air conditioner can be operated with the use of thestorage battery 723 including the storage battery electrode of oneembodiment of the present invention as an uninterruptible power supplyeven when electric power cannot be supplied from a commercial powersupply due to power failure or the like.

Note that although the split-type air conditioner including the indoorunit and the outdoor unit is illustrated in FIG. 8 as an example, thestorage battery including the storage battery electrode of oneembodiment of the present invention can be used in an air conditioner inwhich the functions of an indoor unit and an outdoor unit are integratedin one housing.

In FIG. 8, an electric refrigerator-freezer 730 is an example of anelectric device including a storage battery 734 including the storagebattery electrode of one embodiment of the present invention.Specifically, the electric refrigerator-freezer 730 includes a housing731, a door for a refrigerator 732, a door for a freezer 733, and thestorage battery 734. The storage battery 734 is provided in the housing731 in FIG. 8. The electric refrigerator-freezer 730 can receiveelectric power from a commercial power supply. Alternatively, theelectric refrigerator-freezer 730 can use electric power stored in thestorage battery 734. Thus, the electric refrigerator-freezer 730 can beoperated with the use of the storage battery 734 including the storagebattery electrode of one embodiment of the present invention as anuninterruptible power supply even when electric power cannot be suppliedfrom a commercial power supply due to power failure or the like.

Note that among the electric devices described above, a high-frequencyheating apparatus such as a microwave oven and an electric device suchas an electric rice cooker require high power in a short time. Thetripping of a breaker of a commercial power supply in use of an electricdevice can be prevented by using the storage battery including thestorage battery electrode of one embodiment of the present invention asan auxiliary power supply for supplying electric power which cannot besupplied enough by a commercial power supply.

In addition, in a time period when electric devices are not used,particularly when the proportion of the amount of electric power whichis actually used to the total amount of electric power which can besupplied from a commercial power supply source (such a proportionreferred to as a usage rate of electric power) is low, electric powercan be stored in the storage battery, whereby the usage rate of electricpower can be reduced in a time period when the electric devices areused. For example, in the case of the electric refrigerator-freezer 730,electric power can be stored in the storage battery 734 in night timewhen the temperature is low and the door for a refrigerator 732 and thedoor for a freezer 733 are not often opened or closed. Then, in daytimewhen the temperature is high and the door for a refrigerator 732 and thedoor for a freezer 733 are frequently opened and closed, the storagebattery 734 is used as an auxiliary power supply; thus, the usage rateof electric power in daytime can be reduced.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Embodiment 6

Next, a portable information terminal which is an example of electricdevices will be described with reference to FIGS. 9A to 9C.

FIGS. 9A and 9B illustrate a tablet terminal 800 which can be folded.FIG. 9A illustrates the tablet terminal 800 in the state of beingunfolded. The tablet terminal includes a housing 801, a display portion802 a, a display portion 802 b, a display-mode switching button 803, apower button 804, a power-saving-mode switching button 805, and anoperation button 807.

A touch panel area 808 a can be provided in part of the display portion802 a, in which area, data can be input by touching displayed operationkeys 809. Note that half of the display portion 802 a has only a displayfunction and the other half has a touch panel function. However, thestructure of the display portion 802 a is not limited to this, and allthe area of the display portion 802 a may have a touch panel function.For example, a keyboard can be displayed on the whole display portion802 a to be used as a touch panel, and the display portion 802 b can beused as a display screen.

A touch panel area 808 b can be provided in part of the display portion802 b like in the display portion 802 a. When a keyboard displayswitching button 810 displayed on the touch panel is touched with afinger, a stylus, or the like, a keyboard can be displayed on thedisplay portion 802 b.

Touch input can be performed in the touch panel area 808 a and the touchpanel area 808 b at the same time.

The display-mode switching button 803 allows switching between alandscape mode and a portrait mode, color display and black-and-whitedisplay, and the like. The power-saving-mode switching button 805 allowsoptimizing the display luminance in accordance with the amount ofexternal light in use which is detected by an optical sensorincorporated in the tablet terminal. In addition to the optical sensor,other detecting devices such as sensors for determining inclination,such as a gyroscope or an acceleration sensor, may be incorporated inthe tablet terminal.

Although the display area of the display portion 802 a is the same asthat of the display portion 802 b in FIG. 9A, one embodiment of thepresent invention is not particularly limited thereto. The display areaof the display portion 802 a may be different from that of the displayportion 802 b, and further, the display quality of the display portion802 a may be different from that of the display portion 802 b. Forexample, one of the display portions 802 a and 802 b may display higherdefinition images than the other.

FIG. 9B illustrates the tablet terminal 800 in the state of beingclosed. The tablet terminal 800 includes the housing 801, a solar cell811, a charge/discharge control circuit 850, a battery 851, and a DC-DCconverter 852. FIG. 9B illustrates an example where the charge/dischargecontrol circuit 850 includes the battery 851 and the DC-DC converter852. The storage battery including the storage battery electrode of oneembodiment of the present invention, which is described in the aboveembodiment, is used as the battery 851.

Since the tablet terminal 800 can be folded, the housing 801 can beclosed when the tablet terminal is not in use. Thus, the displayportions 802 a and 802 b can be protected, which permits the tabletterminal 800 to have high durability and improved reliability forlong-term use.

The tablet terminal illustrated in FIGS. 9A and 9B can also have afunction of displaying various kinds of data (e.g., a still image, amoving image, and a text image), a function of displaying a calendar, adate, the time, or the like on the display portion, a touch-inputfunction of operating or editing data displayed on the display portionby touch input, a function of controlling processing by various kinds ofsoftware (programs), and the like.

The solar cell 811, which is attached to a surface of the tabletterminal, can supply electric power to a touch panel, a display portion,an image signal processor, and the like. Note that the solar cell 811can be provided on one or both surfaces of the housing 801 and thus thebattery 851 can be charged efficiently.

The structure and operation of the charge/discharge control circuit 850illustrated in FIG. 9B will be described with reference to a blockdiagram of FIG. 9C. FIG. 9C illustrates the solar cell 811, the battery851, the DC-DC converter 852, a converter 853, switches SW1 to SW3, andthe display portion 802. The battery 851, the DC-DC converter 852, theconverter 853, and the switches SW1 to SW3 correspond to the charge anddischarge control circuit 850 in FIG. 9B.

First, an example of operation in the case where electric power isgenerated by the solar cell 811 using external light will be described.The voltage of electric power generated by the solar cell is raised orlowered by the DC-DC converter 852 so that the electric power can have avoltage for charging the battery 851. When the display portion 802 isoperated with the electric power from the solar cell 811, the switch SW1is turned on and the voltage of the electric power is raised or loweredby the converter 853 to a voltage needed for operating the displayportion 802. In addition, when display on the display portion 802 is notperformed, the switch SW1 is turned off and the switch SW2 is turned onso that the battery 851 may be charged.

Although the solar cell 811 is described as an example of powergeneration means, there is no particular limitation on the powergeneration means, and the battery 851 may be charged with any of theother means such as a piezoelectric element or a thermoelectricconversion element (Peltier element). For example, the battery 851 maybe charged with a non-contact power transmission module capable ofperforming charging by transmitting and receiving electric powerwirelessly (without contact), or any of the other charge means used incombination.

It is needless to say that one embodiment of the present invention isnot limited to the electric device illustrated in FIGS. 9A to 9C as longas the electric device is equipped with the storage battery includingthe storage battery electrode of one embodiment of the presentinvention, which is described in the above embodiment.

Embodiment 7

Further, an example of the moving object which is an example of theelectric devices will be described with reference to FIGS. 10A and 10B.

The storage battery described in the above embodiment can be used as acontrol battery. The control battery can be externally charged byelectric power supply using a plug-in technique or contactless powerfeeding. Note that in the case where the moving object is an electricrailway vehicle, the electric railway vehicle can be charged by electricpower supply from an overhead cable or a conductor rail.

FIGS. 10A and 10B illustrate an example of an electric vehicle. Anelectric vehicle 860 is equipped with a battery 861. The output of theelectric power of the battery 861 is adjusted by a control circuit 862and the electric power is supplied to a driving device 863. The controlcircuit 862 is controlled by a processing unit 864 including a ROM, aRAM, a CPU, or the like which is not illustrated.

The driving device 863 includes a DC motor or an AC motor either aloneor in combination with an internal-combustion engine. The processingunit 864 outputs a control signal to the control circuit 862 based oninput data such as data on operation (e.g., acceleration, deceleration,or stop) of a driver or data during driving (e.g., data on an upgrade ora downgrade, or data on a load on a driving wheel) of the electricvehicle 860. The control circuit 862 adjusts the electric energysupplied from the battery 861 in accordance with the control signal ofthe processing unit 864 to control the output of the driving device 863.In the case where the AC motor is mounted, although not illustrated, aninverter which converts direct current into alternate current is alsoincorporated.

The battery 861 can be charged by external electric power supply using aplug-in technique. For example, the battery 861 is charged through apower plug from a commercial power supply. The battery 861 can becharged by converting the supplied power into DC constant voltage havinga predetermined voltage level through a converter such as an AC-DCconverter. The use of the storage battery including the storage batteryelectrode of one embodiment of the present invention as the battery 861can be conducive to an increase in battery capacity, leading to animprovement in convenience. When the battery 861 itself can be morecompact and more lightweight as a result of improved characteristics ofthe battery 861, the vehicle can be lightweight, leading to an increasein fuel efficiency.

Note that it is needless to say that one embodiment of the presentinvention is not limited to the electric device described above as longas the storage battery of one embodiment of the present invention isincluded.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Example 1

One embodiment of the present invention is specifically described belowwith an example. Note that one embodiment of the present invention isnot limited to the following example.

<Materials & Equipment>

Materials and equipment used in this example will be described below.

As graphite, BF-40AK manufactured by Chuetsu Graphite Works Co., Ltd.was used. Lithium carbonate Li₂CO₃ (99.99%, anhydrous) purchased fromKojundo Chemical Laboratory Co., Ltd. was used. Ammonium dihydrogenphosphate NH₄H₂PO₄ (99%) and D(+)-glucose (98%, referred to as glucosebelow) were purchased from Kishida Chemical Co., Ltd. As iron oxalate,FeC₂O₄.2H₂O (98%, dihydrate) was purchased from Kishida Chemical Co.,Ltd. L-ascorbic acid (99.6%, referred to as an ascorbic acid below) waspurchased from Wako Pure Chemical Industries, Ltd. PVdF No. 7300 (a 5 wt% NMP solution) manufactured by Kureha Corporation was used. As apositive electrode current collector, an aluminum foil covered with alayer substantially formed of carbon black was used. As a referencepositive electrode current collector, an aluminum foil covered byapplication of a polymer-including carbon black dispersion liquid wasused. As a negative electrode current collector, a rolled copper foilwas used. A mixed solvent of ethylene carbonate (EC) and diethylcarbonate (DEC) (EC:DEC=1 vol:1 vol, battery grade) including 1M ofLiPF₆ purchased from Kishida Chemical Co., Ltd was used.

For spray drying, a mini spray dryer B-290 manufactured by Nihon BUCHIK.K. was used. For the adhesive tape 180° peeling test, EZGraphmanufactured by Shimadzu Corporation was used. For electrode resistancemeasurement, Loresta GP with PSP-type probes manufactured by MitsubishiChemical Analytech, Co., Ltd. was used.

<Synthesis of Graphene Oxide>

Graphene oxide was synthesized by the following method. First, while 20g of graphite and 690 mL of a concentrated sulfuric acid (96%) werestirred in an ice bath, 75 g of potassium permanganate was slowly addedthereto. Then, stirring was performed at 25° C. for 4.5 hours to giveReaction Liquid 1. Next, Reaction Liquid 1 was slowly added to 1380 mLof pure water in an ice bath and was diluted. Next, diluted ReactionLiquid 1 was stirred in an oil bath at ca. 95° C. for 15 minutes tocause a reaction, and then 54 mL of a hydrogen peroxide solution(concentration: 30 wt %) was added in a water bath (room temperature) toinactivate unreacted potassium permanganate. After a solid was collectedby centrifugation, cleaning and drying were performed to obtain grapheneoxide powder. Here, cleaning was performed by repeating the processconsisting of dilution and dispersion with pure water, centrifugation,and collection of a precipitate. Drying was performed with the spraydryer.

<Synthesis of Active Material Particle LiFePO₄>

Acetone (dehydrated) was added to Li₂CO₃, FeC₂O₄.2H₂O, and NH₄H₂PO₄(molar ratio of 1:2:2), and crushing and mixing were performed with amedium stirring mill. A solvent of the mixture was volatilized andbaking was performed at 350° C. for 10 hours in a nitrogen atmosphere togive Material 1. Then, glucose (10 wt % with respect to Material 1) andacetone were added to Material 1 and mixed with a wet medium mill, and asolvent was volatilized to give Material 2. Material 2 was baked at 600°C. in a nitrogen atmosphere for 10 hours to give Material 3. Material 3was microparticulated using acetone with a wet medium mill, that is,dispersed, and a solvent was volatilized to give Material 4. Material 4was used as active material particles LiFePO₄. Note that the crystallitesize and specific surface area of Material 4 were found by XRD analysisto be ca. 80 nm and 27 m²/g, respectively.

FIGS. 11A and 11B show the particle size distribution of active materialparticles measured in a water solvent. FIG. 11A shows frequencydistribution, and FIG. 11B shows cumulative distribution. The 90%particle diameter (the particle diameter when the cumulative percentageis 90% in the cumulative distribution) was 2.708 μm.

<Electrode Formation>

NMP was added to the active material particles LiFePO₄, graphene oxide,and a PVdF solution, and mixed in a thin-film rotary high-speed stirrer.The weight ratio of the materials except for the solvent NMP was asfollows: LiFePO₄:graphene oxide:PVdF=94.2:0.8:5.0. The obtained slurrymixture was applied onto a positive electrode current collector by ablade method, and dried with a hot wind at temperatures of 65° C. to 75°C.; thus, Electrode 1 including a positive electrode active materiallayer was formed. Furthermore, a slurry mixture which is an NMP solutionincluding active material particles LiFePO₄, graphene oxide, and PVdF isapplied to a reference positive electrode current collector by a blademethod, and dried with a hot wind; thus, Electrode 2 including apositive electrode active material layer was formed. The weight ratiofor materials of Electrode 2 is the same as that for Electrode 1.

Electrode 1 was immersed in an NMP solution including 10 vol % of water,77 mM of an ascorbic acid, and 75 mM of lithium hydroxide, and areaction was caused at 60° C. for 1 hour. Then, the reacted Electrode 1(referred to as Electrode 1 a below) was cleaned several times withethanol. Then, the cleaned Electrode 1 a was dried and subjected to heattreatment at 170° C. for 10 hours under vacuum. The heated Electrode 1 awas pressed; thus, Electrode A was formed. By shaping Electrode A into acircular shape with a diameter of 12 mm by punching, Positive ElectrodeA was formed.

Electrode 1 was immersed in an NMP solution including 10 vol % of water,77 mM of an ascorbic acid, and 75 mM of lithium hydroxide, and areaction was caused at 60° C. for 1 hour. Then, the reacted Electrode 1(referred to as Electrode 1 b below) was cleaned several times with purewater. Then, the cleaned Electrode 1 b was dried and subjected to heattreatment at 170° C. for 10 hours under vacuum. The heated Electrode 1 bwas pressed; thus, Electrode B was formed. By shaping Electrode B into acircular shape with a diameter of 12 mm by punching, Positive ElectrodeB was formed.

Electrode 1 was immersed in an aqueous solution including 77 mM of anascorbic acid and 75 mM of lithium hydroxide, and a reaction was causedat 80° C. for 5 minutes. Then, the reacted Electrode 1 (referred to asElectrode 1 c below) was cleaned by being immersed in pure water at 60°C. for 5 minutes. Then, the cleaned Electrode 1 c was dried andsubjected to heat treatment at 170° C. for 10 hours under vacuum. Theheated Electrode 1 c was pressed; thus, Electrode C was formed. Byshaping Electrode C into a circular shape with a diameter of 12 mm bypunching, Positive Electrode C was formed.

Electrode 1 was immersed in an aqueous solution including 77 mM of anascorbic acid and 75 mM of lithium hydroxide, and a reaction was causedat 80° C. for 15 minutes. Then, the reacted Electrode 1 (referred to asElectrode 1 d below) was cleaned by being immersed in pure water at 60°C. for 5 minutes. Then, the cleaned Electrode 1 d was dried andsubjected to heat treatment at 170° C. for 10 hours under vacuum. Theheated Electrode 1 d was pressed; thus, Electrode D was formed. Byshaping Electrode D into a circular shape with a diameter of 12 mm bypunching, Positive Electrode D was formed.

Electrode 1 was immersed in an aqueous solution including 77 mM of anascorbic acid and 75 mM of lithium hydroxide, and a reaction was causedat 80° C. for 1 hour. Then, the reacted Electrode 1 (referred to asElectrode 1 e below) was cleaned by being immersed in pure water at 60°C. for 5 minutes. Then, the cleaned Electrode 1 e was dried andsubjected to heat treatment at 170° C. under vacuum. The heatedElectrode 1 e was pressed; thus, Electrode E was formed. By shapingElectrode E into a circular shape with a diameter of 12 mm by punching,Positive Electrode E was formed.

Electrode 1 was immersed in an aqueous solution including 77 mM of anascorbic acid and 75 mM of lithium hydroxide, and a reaction was causedat 60° C. for 15 minutes. Then, the reacted Electrode 1 (referred to asElectrode if below) was cleaned by being immersed in pure water at 60°C. for 5 minutes. Then, the cleaned Electrode if was dried and subjectedto heat treatment at 170° C. for 10 hours under vacuum. The heatedElectrode if was pressed; thus, Electrode F was formed. By shapingElectrode F into a circular shape with a diameter of 12 mm by punching,Positive Electrode F was formed.

Electrode 2 was immersed in an aqueous solution including 77 mM of anascorbic acid and 75 mM of lithium hydroxide, and a reaction was causedat 80° C. In this case, the active material layer and the currentcollector were separated from each other in a reducing solution.Therefore, Electrode 2 was not used any more.

In Positive Electrode A, the weight of the active material layer was13.7 mg, and the thickness of the active material layer was 66 urn. Whencalculated without considering the change in weight from graphene oxideto RGO, that is, with the assumption that the percentage of the activematerial in the active material layer is 94.2 wt %, the area is 1.13cm², the active material weight is 12.9 mg, and the active materiallayer density is 1.84 g/cm³.

In Positive Electrode B, the weight of the active material layer was 133mg, and the thickness of the active material layer was 66 μm. Whencalculated without considering the change in weight from graphene oxideto RGO, that is, with the assumption that the percentage of the activematerial in the active material layer is 94.2 wt %, the area is 1.13cm², the active material weight is 12.3 mg, and the active materiallayer density is 1.87 g/cm³.

In Positive Electrode C, the weight of the active material layer was13.1 mg, and the thickness of the active material layer was 62 μm. Whencalculated without considering the change in weight from graphene oxideto RGO, that is, with the assumption that the percentage of the activematerial in the active material layer is 94.2 wt %, the area is 1.13cm², the active material weight is 12.6 mg, and the active materiallayer density is 1.91 g/cm³.

In Positive Electrode D, the weight of the active material layer was13.3 mg, and the thickness of the active material layer was 62 μm. Whencalculated without considering the change in weight from graphene oxideto RGO, that is, with the assumption that the percentage of the activematerial in the active material layer is 94.2 wt %, the area is 1.13cm², the active material weight is 12.9 mg, and the active materiallayer density is 1.89 g/cm³.

In Positive Electrode E, the weight of the active material layer was13.1 mg, and the thickness of the active material layer was 65 μm. Whencalculated without considering the change in weight from graphene oxideto RGO, that is, with the assumption that the percentage of the activematerial in the active material layer is 94.2 wt %, the area is 1.13cm², the active material weight is 12.3 mg, and the active materiallayer density is 1.78 g/cm³.

In Positive Electrode F, the weight of the active material layer was13.2 mg, and the thickness of the active material layer was 62 μm. Whencalculated without considering the change in weight from graphene oxideto RGO, that is, with the assumption that the percentage of the activematerial in the active material layer is 94.2 wt %, the area is 1.13cm², the active material weight is 12.5 mg, and the active materiallayer density is 1.89 g/cm³.

<Battery Characteristics>

Positive Electrodes A to F, Li metal as a negative electrode, an EC/DECelectrolytic solution (EC:DEC 1 vol:1 vol) including 1M of LiPF₆ as anelectrolyte, and porous polypropylene as a separator were used tofabricate 2032-type coin batteries which are referred to as CoinBatteries A to F.

The charge and discharge characteristics of Coin Batteries A to F (25°C.) were measured. Specifically, after charging at a constant current(approximately 0.4 mA) and a constant voltage (4.3 V) is performed,discharging at a constant current was performed. In each of the coinbatteries, the charge rate per unit weight of the positive electrodeactive material was set at 0.2 C, and charging was performed.

Here, a charge rate and a discharge rate will be described. For example,in the case of charging a secondary battery with a certain capacityX[Ah] at a constant current, a charge rate of 1 C means the currentvalue I[A] with which charging is completed in 1 hour, and a charge rateof 0.2 C means I/5 [A] (i.e., the current value with which charging iscompleted in 5 hours). Similarly, a discharge rate of 1 C means thecurrent value I[A] with which discharging is completed in 1 hour, and adischarge rate of 0.2 C means I/5 [A] (i.e., the current value withwhich discharging is completed in 5 hours).

In this example, the current value at the time of charging was set withthe assumption that the capacity of the positive electrode activematerial was 170 mAh/g. In other words, the charge rate 0.2 C means acharge with a current value of 34 mA/g. The discharge rate 0.2 C means adischarge with a current value of 34 mA/g. The discharge rate 1 C meansa discharge with a current value of 170 mA/g. The discharge rate 10 Cmeans a discharge with a current value of 1.7 A/g.

The discharge curves at discharge rates 0.2 C, 1 C, and 10 C of CoinBatteries A, B, and E are shown in FIG. 12. The shapes of the dischargecurves are not largely different from each other. Although the dischargecurves of Coin Batteries A and B are the same for the most part, thedischarge capacity of Coin Battery E at especially a high rate of 10 Cis larger than those of Coin Batteries A and B.

The discharge capacities per unit weight of the positive electrodeactive material at discharge rates 1 C and 10 C in Coin Batteries A to Fare shown in Table 1.

TABLE 1 Reduction Cleaning Capacity [mAh/g] No. Solvent ConditionsSolvent 1 C 10 C A NMPaq 60° C. EtOH 152 55 60 min B NMPaq 60° C. H₂O151 56 60 min C H₂O 80° C. H₂O 152 50  5 min D H₂O 80° C. H₂O 151 57 15min E H₂O 80° C. H₂O 155 65 60 min F H₂O 60° C. H₂O 152 56 15 min

By comparing Coin Batteries C to E, which are different from each otheronly in the reduction time in formation of the positive electrode, itwas found that the discharge capacity depends on the reduction time.Moreover, Coin Batteries D and F with the reduction time of only 15minutes showed discharge capacities equivalent to those of CoinBatteries A and B with the reduction time of 1 hour.

<Peeling Force Test>

The adhesive tape 180° peeling test (related standard: ISO29862:2007) ofElectrodes A and B was performed. Specifically, the active materiallayer was fixed, and the force necessary for the peeling (peeling force)was measured by peeling the current collector off from the fixed activematerial layer at the 180° direction. As described above, Electrode Aand Electrode B use different cleaning solvents after the chemicalreduction.

The peeling forces of Electrode A and Electrode B were 0.97 N and 130 N,respectively. This suggests that immersion in pure water increases thepeeling force and thus makes it difficult to peel the current collectorfrom the active material layer.

<ToF-SIMS Analysis>

Electrode A and Electrode B were analyzed by Time-of-flight secondaryion mass spectrometry (ToF-SIMS). As described above, Electrode A andElectrode B use different cleaning solvents after the chemicalreduction. Note that the ascorbic acid and a derivative thereof were notable to be analyzed by ToF-SIMS owing to interfering ions.

FIGS. 13A to 13D and FIGS. 14A to 14D show ToF-SIMS analysis results.More peaks of C₃H₂F₃ ions and C₃HF₄ ions were detected in Electrode Athan in Electrode B. These ions are derived from PVdF. More peaks of Clions, which are halogen ions, and C₇H₇ ions, which are hydrocarbon ions,were detected in Electrode B than in Electrode A. Although not shown inthe data, in Electrode 8, detected peaks of C₃H₂F₃ ions and C₃HF₄ ionsvaried in the detected amount depending on the measured area. Theseresults might indicate the occurrence of PVdF modification, that is,elimination of hydrogen fluoride. More specifically, a reason for thedetection of more Cl ions might be that the impurity Cl was caught afterelimination of hydrogen fluoride. The elimination of hydrogen fluorideseems to partly form a polyene structure. In addition, the formedpolyene structure is presumably cyclized to form an aromatic ringstructure. The polyene structure might be detected as a cyclizedaromatic ring in ToF-SIMS. Peaks of Li₂OH ions and Li₃O ions weredetected both in Electrode A and Electrode B. The number of detectedpeaks of Li₃O ions in Electrode A was slightly larger than that inElectrode B. This can be considered as the effect of the cleaning withwater, but the difference was not so significant. This suggests thepossibility of the influence of PVdF modification, rather than merelythe effect of the cleaning.

<Electrode Resistance Measurement>

The sheet resistances of Electrodes A, E, and F were measured by afour-terminal four-probe method. Specifically, the active material layerwas peeled from the current collector, and the sheet resistance of theactive material layer on an insulator was measured. As described above,the reduction solvent and the cleaning conditions are different betweenElectrode A and Electrodes E and F, and the reduction temperature andtime are different between Electrode E and Electrode F.

The sheet resistance of Electrodes A, E, and F were 450 Ω·cm, 11 ∩·cm,290 Ω·cm, respectively. In other words, the resistance of Electrode Ereduced well in the aqueous solution was 10 times or more lower thanthat of Electrode A. As for battery characteristics, as described above,the discharge capacity at a high rate (10 C) in Electrode E is higherthan that in Electrode A, which shows a possibility that the batterycharacteristics can be related to the sheet resistance. In Electrode 1,a material that can serve as a conductive additive is only grapheneoxide. Thus, the reduction in the aqueous solution and cleaning withpure water can lower the resistance of graphene (RGO). This might bebecause of a reduction in the resistance of the graphene flakesthemselves or a reduction in the resistance between graphene flakes. Inthe former case, the electrode including graphene is increased inperformance by being immersed in an aqueous solution. This suggests thatthe performance is increased not only by reduction with a reducing agentbut also immersion in an aqueous solution after the reduction with thereducing agent. The latter case is presumably owing to an improvement inthe contact state; in other words, the contact resistance betweenparticles (active material particles including RGO) is lowered becausebonding between the particles and a binder that bonds the particles isimproved and thus bonding between the particles becomes stronger. If itis considered in this way, in the case where a metal foil covered with acarbon particle is used as the current collector, the contact resistancebetween the current collector and the active material layer is alsoaffected. Note that the contact resistance between the current collectorand the active material layer is initially low enough and normallynegligible. For storage or after long-term use, peeling between thecurrent collector and the active material layer is observed in somecases, and the contact resistance might have an influence on cycleperformance and calendar life (storage characteristics).

CONCLUSION

In this example, the influence of the combination of the modified PVdFand graphene was examined, and it was suggested that a polyene structureor an aromatic ring structure is formed by PVdF modification and thisPVdF modification is caused by the contact with an aqueous solutionincluding pure water.

This application is based on Japanese Patent Application serial no.2014-111254 filed with Japan Patent Office on May 29, 2014, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A method for forming an electrode comprising:forming a mixture comprising an active material particle, grapheneoxide, and a poly(vinylidene fluoride); providing the mixture over acurrent collector; and dehydrofluorinating the poly(vinylidene fluoride)by treating the mixture in a solution comprising a polar solvent and areducing agent.
 2. The method for forming an electrode according toclaim 1, wherein the polar solvent comprises at least one of water,methanol, ethanol, acetone, tetrahydrofuran, dimethylformamide,1-methyl-2-pyrrolidone and dimethyl sulfoxide.
 3. The method for formingan electrode according to claim 1, wherein the reducing agent is amaterial having a LUMO level of higher than or equal to −5.0 eV andlower than or equal to −3.8 eV.
 4. The method for forming an electrodeaccording to claim 1, wherein the reducing agent comprises at least oneof ascorbic acid, hydrazine, dimethyl hydrazine, hydroquinone,tetra-butyl ammonium bromide, NaBH₄ and N,N-diethylhydroxylamine.
 5. Themethod for forming an electrode according to claim 1, wherein a polyenestructure or an aromatic ring structure is formed in the poly(vinylidenefluoride) by the dehydrofluorination.
 6. A method for forming anelectrode comprising: forming a layer over a metal layer, the layercomprising a carbon particle; forming a mixture comprising an activematerial particle and a poly(vinylidene fluoride); providing the mixtureover the layer; and dehydrofluorinating the poly(vinylidene fluoride) bytreating the mixture in a solution comprising a polar solvent and areducing agent.
 7. The method for forming an electrode according toclaim 6, wherein the polar solvent comprises at least one of water,methanol, ethanol, acetone, tetrahydrofuran, dimethylformamide,1-methyl-2-pyrrolidone and dimethyl sulfoxide.
 8. The method for formingan electrode according to claim 6, wherein the reducing agent is amaterial having a LUMO level of higher than or equal to −5.0 eV andlower than or equal to −3.8 eV.
 9. The method for forming an electrodeaccording to claim 6, wherein the reducing agent comprises at least oneof ascorbic acid, hydrazine, dimethyl hydrazine, hydroquinone,tetra-butyl ammonium bromide, NaBH₄ and N,N-diethylhydroxylamine. 10.The method for forming an electrode according to claim 6, wherein apolyene structure or an aromatic ring structure is formed in thepoly(vinylidene fluoride) by the dehydrofluorination.